Xen is a hypervisor, which is a software that allows running multiple virtual machines on one physical computer. It was established at the University of Cambridge and is now developed by the U.S. company Citrix Systems.

Technical details

Xen is a hypervisor (also Virtual Machine Monitor ( VMM) is called ), which runs directly on hardware. Xen can run multiple operating systems in virtual machines, known as domains. For these operating systems are not the hypervisor other domains "visible". In principle, the procedure is similar to virtual memory and processes: The virtual memory, each process (in this case the virtual system ) use the memory as if it were the only process that is executed by the operating system. More specifically, the hypervisor assigns the virtual system to portions of the entire main memory. These appear to the system as a contiguous virtual address space, so as the physical memory to a non-virtual system appears. It can be used by the virtual machine accordingly and exclusive.

Of special significance is the first domain that is launched from Xen: This domain is privileged and is used to interact with the actual hypervisor. The privileged domain called Dom0 can start other domains, stop, and manage. To this end, this administration functionality must be built into the operating system that runs in the Dom0.

In order to be completely transparent for the unprivileged domains, often called DomU, Xen requires a main processor with the instruction set extension Secure Virtual Machine, such as Intel VT or AMD -V. With this hardware, the operating systems that run in the domains do not need to be adjusted - they do not " notice " that they share the hardware in reality with other systems. This mode is called full virtualization or hardware virtual machine ( HVM ). Is virtualized on other hardware, have the respective kernel full hardware access and that due to erroneous or malicious code on each foreign resources (eg memory) access, which is not desirable for reasons of stability and security of the whole.

The efficiency of virtualized systems can be enhanced by support for the operation will be integrated as DomU in the operating system. This approach is referred to as paravirtualization and requires a modification of the system to run in a DomU.

Supported Operating Systems

The Linux kernel is version 2.6.21 from the conditions for the operation under any hypervisor in the form of so-called paravirt ops ready. Since version 2.6.23, limited support for running under Xen is integrated. However, this basic support supports numerous possibilities of Xen not, for example, the (dynamic ) passing through of PCI devices or dynamic memory magnifications.

In Linux distributions, openSUSE, version 9.3 and version 4 in Fedora Xen is already integrated. Also included Xen is in the Novell / SUSE Linux Enterprise Server ( SLES ) version 10 and Red Hat Enterprise Linux 5 ( RHEL5 ). Gentoo Linux also offers a modified Xen kernel, however it is still marked as unstable ( unstable). Since version 4 (etch), Debian contains a Xen kernel. Xen 3.1 held in December 2007, entry into Univention Corporate Server 2.0 and has been part of the enterprise distribution based on Debian GNU / Linux.

In versions to Xen 3.x "official " Linux kernel source Xen is suitable for operation as Dom0 or DomU as full native only, which is present only in version Due to the active development of the Linux kernel itself can not be applied without considerable effort to a recent kernel patch for this version. So the elaborate practice to scale Xen patches to newer versions, such as Debian has recently been discontinued.

Since Xen 4.0 supports Xen by default, the default kernel option pvops dom0 kernel for its in kernel version 2.6.31.x. Further, a Long-Term Support version (LTS ) is available for dom0 kernel under pvops and the Linux kernel 2.6.32.x. Nevertheless, the compatibility with the previous 2.6.18 kernel will continue to receive and there are other Xen patches for this release as well as for some kernel patches forward (such as the kernel of RHEL 5.x) planned.

NetBSD 2.0 supported Xen 1.2 host and guest and Xen 2.0 as a guest which version of NetBSD 3.1 supports Xen 2.0 completely, so as host and guest and Xen 3.0 as a guest. Since NetBSD 4.0 Xen 3.0 is fully supported.

Sun has Xen beginning of October 2007 with Nevada build 75 fully integrated under the name xVM in OpenSolaris, before there were trials.

On the Novell BrainShare Conference in 2005 introduced Novell NetWare on a port of Xen before.

At one port of ReactOS on Xen is working since 2005.

Supporters and cooperation

Among the supporters of Xen include global IT companies - even directly strongly with each other competing companies come together under this umbrella, including: Microsoft and Sun / Oracle, Intel and AMD, IBM, HP, Red Hat and Novell / SUSE.

The open source Xen software was originally developed at the University of Cambridge. The developers have founded XenSource with a company that will make Xen the industry standard. The company XenSource was acquired in August 2007 for 500 million U.S. dollars by company Citrix Systems.

Early 2013 XAPI was then transferred together with Xen.org back to the Xen Project, which operates under the umbrella of the Linux Foundation. Xapi is now therefore a sub-project of the Xen Project.


  • Support for Intel VT for HVM guests.
  • Support of the IA -64 architecture.

Up to version 3.0.4 also following features were added:

  • Support AMD virtualization technology.
  • Support the PowerPC architecture.
  • Support for a graphical framebuffer for paravirtualized guests.