Xen vs. KVM – Comparison of Hypervisors

When we talk about the virtualization concept, among the terms that will always come up is hypervisor technology. A hypervisor is used by individuals or organizations that intend to merge their server space or operate additional servers without relying on the host server. Hypervisors execute virtualization in a way that ascertains central management of data, and this is precisely what Xen and KVM hypervisors do.

Xen is a general-public-license hypervisor that was released in 2003. Xen performed excellently, and it seemed to be Linux’s best virtualization platform then, not until 2007, when KVM started proving itself as a worthy alternative.

Things in the virtualization world are evolving; hence the roles of hypervisors keep expanding. Keeping up with the recent developments in the virtualization world might be challenging, and you might have difficulty deciding which one to choose between these two hypervisors. You’re at the right place; read on, and your confusion will be settled.


Xen is an open-source, type-1 hypervisor that can be installed directly on a computer without the intervention of a host operating system. Being a type-1 hypervisor, Xen controls, oversees, and manages every aspect of a  computer( hardware, peripheral, and input/output resources) without any intermediary.

Xen is compatible with multiple renditions of the same or different operating systems. It has native support for most operating systems, including the Linux kernel; hence it is available in all Linux distributions. Additionally, Xen is an essential component of guest virtual machines;  it must be installed on these virtual machines so that they can access hardware components.

In essence, Xen is an open-source hypervisor that allows the simultaneous development, execution, and management of multiple virtual machines on one physical computer.

Xen offers two types of virtualization: paravirtualization and full virtualization.

In paravirtualization, the virtualized operating system operates on a tweaked version of the OS. As a result, the tweaked operating system becomes aware that it is virtualized. This permits more efficient interaction between the OS and the physical hardware since the hardware devices are addressed directly. However, the fact that the functionality of paravirtualization relies on modified guest OS is a downside since most vendors don’t provide it.

Xen also offers full virtualization, a mode where all virtualization extensions require the CPU’s support. Here, the unmodified operating systems can efficiently instruct the hardware because of this support. Full virtualization often comes with performance drawbacks because complete emulation usually requires more processing resources and overhead resources from the hypervisor.

Xen functions well on x86, x86_64, Itanium, and ARM frameworks. Furthermore, Xen can operate Linux, Windows, Solaris, and some Berkeley Software Distributions(BSDs) as guests on their supported CPU frameworks. Moreover, Xen is used by several companies, mainly by Citrix. But it is also adopted by Oracle for Oracle VM and by others.

Xen can perform full virtualization on systems compatible with virtualization extensions and still retain its ability to work as a hypervisor on guest machines that lack these virtualization extensions. Hence, Xen can run multiple guest operating systems, each OS running in Xen’s virtual machine or domain. Upon its installation, Xen automatically creates the first domain called Domain 0. Domain 0 is responsible for managing the system, and it performs tasks like managing virtual devices for each virtual machine, migrating virtual machines, building additional domains, etc.

Pros of  Xen Hypervisor

  • Xen is a perfect fit for small organizations since it is available for free and manages resources efficiently.
  • Xen allows real-time transition of virtual machines. As a result, it maintains system operations regardless of background hardware changes.
  • Since Xen is an open-source hypervisor, users do not need to worry about vendor lockdown issues.
  • Xen provides users with a graphical user interface with an amazing environment that makes interaction easy.
  • As a hypervisor, Xen can securely transfer a virtual server from one physical server to another physical server.
  • Xen possesses a console that can implement upgrades to the hypervisor pool.
  • Xen has a wider community of third-party availability, backup, storage handling, capacity planning, performance supervision, process control, and other management strategies.

Cons of  Xen Hypervisor

Even though Xen is one of the best hypervisors, it still has some downsides.

  • Xen hypervisor does not support USB devices.
  • The free version of the Xen hypervisor offers limited services. One has to opt for the paid version to enjoy Xen’s full benefits.
  • Increasing the host’s storage space is arduous
  • A new user can not easily do the upgrade process.
  • Less stable virtual networks.


Kernel-based Virtual Machine (KVM) is a type-1, open source virtualization technology that is Linux based. KVM allows users to convert Linux into a type-1 hypervisor, thereby allowing a host machine to operate multiple guest virtual machines, which are segregated virtual environments. KVM is compatible with most Linux operating systems, including Ubuntu, SUSE, and Red Hat Enterprise Linux. It also supports other operating systems, such as Windows.

Like other Linux kernel hypervisors, KVM has all the essential operating system-level components required to operate virtual machines. These include a memory manager, process scheduler, input/output (I/O) stack, device drivers, security manager, network stack, etc.  Each virtual machine is operated as a regular Linux process, possessing dedicated virtual hardware similar to a network card, graphics adapter, CPU(s), memory, and disks.

In addition to Paravirtualization and full virtualization, KVM also offers hardware-assisted paravirtualization. In this case, the supported hardware functions with the processor. By implication, efficiency is increased, and the amount of modifications required for guest systems is reduced.

Basically, a KVM host doesn’t operate alone. On the Linux kernel, KVM operates along with two KVM kernel modules (the kvm.ko module and either kvm-intel.ko or kvm-amd.ko). As a result of KVM’s tight kernel integration  that entails the input/output  connectivity with kernel block and network drivers, KVM provides its guests with more seamless access to all the complex hardware and networking profiles that they might be faced with.

Due to the incorporation of hardware virtualization extensions that are required for KVM development into modern CPU designs, KVM guests can securely access only those hardware resources they need without the need to worry about leakage to the larger system.

When using KVM, your host OS has to be Linux. Nevertheless, it functions seamlessly on Linux, Windows, Solaris, and BSD guests. Additionally, KVM also operates on x86 and x86-64 systems with hardware supporting virtualization extensions. This implies that KVM isn’t compatible with older CPUs made before the establishment of virtualization extensions, and it doesn’t function on modern CPUs, such as Intel’s Atom CPUs, that do not have virtualization extensions.

Pros of KVM Hypervisor

Presently, KVM seems to be the users’ favorite when it comes to virtualization. Here are some benefits KVM offers:

  • KVM offers commendable security as it is Linux-based and has the entire benefits of Linux’s advanced security features.
  •  Being an open-source hypervisor, KVM allows users to use and modify its features based on personal or business requirements.
  • KVM possesses Linux’s memory management, enabling KVM to be more aware of updates.
  • Also, KVM offers a swift migration time. This implies that machines can be migrated offline and online without the risk of data loss or downtime. This feature goes a step further by allowing users to migrate to a server with a different CPU framework and even channel it through an external service, such as Secure Shell.

Cons of KVM Hypervisor

  • KVM has a sophisticated setup process.
  • KVM doesn’t have sufficient processor support.
  • A built-in CPU virtualization support is lacking on KVM. Hence, its performance is slightly affected.
  • Since the hardware is centralized, there’s a greater risk of losing data in the event of a system failure.

The Differences Between Xen And KVM

Xen can function seamlessly on the hardware of a computer, replacing the operating system. As a result, multiple guest OS can operate simultaneously on the computer’s hardware. KVM has a kernel module called kvm.ko, which utilizes the CPU’s virtualization ability and Linux memory. kvm.ko effectively allocates virtual CPU and memory as a Linux process.
Xen is a virtualization entity that comes under Linux.  KVM is seen as a superficial hypervisor module that is based on the Linux kernel.
Xen’s supporting enterprises include Citrix, Novell, Oracle, Ret Hat (RHEL5), and Virtual Iron. KVM is supported on Redhat, Ubuntu, etc.
Xen has a diverse supporting structure; it supports x86, IA64, and ARM( Advanced RISC Machine)  from AMD(Advanced Micro Devices), Fujitsu, IBM, x86/64 CPU vendors and Intel CPU embedded support, etc. KVM is compatible with any CPU that supports virtualization.
Xen needs to be patched to the kernel  KVM is built into the kernel.


In essence, Virtualization has caused a significant transformation in the IT world and has made deployment easier on every platform. With the introduction of Xen and KVM hypervisors, virtualization gets better.


KVM and Xen are two efficient hypervisors written in C language.

What distinguishes them is that KVM is a virtualization module in the Linux kernel that operates similarly to a hypervisor. At the same time, Xen is a type 1 hypervisor that permits simultaneous running of multiple operating systems on the same computer hardware.

Xen and KVM offer specific benefits, primarily the ability to run multiple operating systems simultaneously on a single physical server and obtain access to network flexibility. An organization’s decision to choose between these two hypervisors depends on the organization’s primary infrastructure, staff resources, and interest in using the cloud.

Last but not least, organizations using XEN or KVM-based virtualization need to remember that they are still vulnerable to ransomware attacks and other disasters. As such, a reliable backup and recovery software like Storware can come in handy.

text written by:

Paweł Piskorz, Presales Engineer at Storware