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Once a server virtualization leader, Xen has faded from the spotlight. But don't let the lack of flashy updates fool you; the Xen open source project is alive and well as the foundation for some of the world's major clouds.
A decade ago when server virtualization started to catch on, the Xen open source project was the leading hypervisor technology. Its paravirtualization approach offered great benefits, even over VMware ESX. Every Linux distribution included a Xen Stack.
Today, virtualization is part of the Linux kernel. KVM features have rapidly evolved, and today most Linux distributions focus purely on the KVM stack -- including Red Hat, which decided to drop Xen support in Red Hat Enterprise Linux 6. Does that mean that Xen has disappeared?
It's true, Xen has seen some difficult moments over the past decade. Citrix's purchase of XenSource -- the company founded by Xen creators -- weakened the Xen open source project and seemed to be the end of Xen. However, Xen is still used as the virtualization technology behind many big projects. Did you know, for instance, that Amazon Web Services uses the Xen hypervisor for all cloud instances?
Linux Foundation to the rescue
Xen has never disappeared. It has, however, dropped off many people's radar. When Red Hat made its move away from Xen in favor of KVM, Xen was already the foundation of many big projects. Not only do major public cloud providers rely on Xen, but some automotive companies rely on Xen because of its reputation for being a very stable hypervisor platform.
The turning point for Xen was in April 2013, when Linux Foundation adopted the Xen open source project. As the organization founded to foster the growth of Linux, the Linux Foundation's embrace of Xen was an important step for the hypervisor's future. The result of that move is that most Linux distributions continue to offer a complete Xen Stack. For example, Oracle and SUSE both have a two-stack policy where the customer can select the hypervisor that best fits their needs. Oracle VM, which can be used as an alternative for VMware vSphere, is built on top of the Xen hypervisor.
The Xen open source project still is the foundation for Citrix XenServer. The involvement of a large commercial vendor such as Citrix helps drive development in the open source project as well.
At home in the Linux kernel
One of the primary reasons that Linux distributions switched to KVM around 2010 was the Linux kernel's lack of native Xen support. To run Xen, it was necessary to run a specific Xen kernel, which had its own development tree and contained many patches on a fixed version of the Linux kernel. Xen is now included in the Linux kernel.
Organizations that use Xen appreciate its technical features. According to many Xen users, the hypervisor is very stable. Xen also offers features still absent in KVM, such as the option to increase VM memory without having to shut down or reboot the VM. Xen also offers better drivers for Windows.
Xen has developed into a very stable virtualization platform that continues to serve as the foundation for many public cloud environments.
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