Red Hat started a trend away from then-frontrunner hypervisor Xen when it switched its default hypervisor to KVM...
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in 2010. Other Linux distributions soon followed suit, and Xen silently faded from relevance. However, recent developments may mark a comeback for the Xen hypervisor, with signs pointing toward a more promising future.
The hypervisor that started it all
Not so long ago, the Xen open source project revolutionized virtualization. With its paravirtualization approach, Xen made virtualization much more efficient and offered better performance than VMware.
But from a technical perspective, Xen was complicated and not included in the Linux kernel. That means that normal kernel upgrades weren't possible, making it difficult to apply security updates. KVM is native to the Linux kernel and used a very simple architecture of just two kernel modules that automatically followed the upgrade path of the Linux kernel.
So although Xen was a leading hypervisor among Linux distributions and boasted better performance and options, KVM soon replaced Xen shortly after it came out in 2007.
The Xen hypervisor suffered another blow from Citrix Systems' purchase of XenSource Inc., the company that offered a commercial platform based on the open source Xen hypervisor. With that purchase, Citrix became more or less the owner of the Xen platform. The perception that Citrix owned Xen hurt the hypervisor's reputation, and some former supporters thought that Citrix's acquisition isolated Xen from the open source community, according to Xen evangelist Russell Pavlicek.
The Xen hypervisor was dealt a third blow when Red Hat purchased Qumranet Inc., the company that employed most of the KVM developers, in 2008. It was two years after that purchase that Red Hat announced KVM would be the only supported virtualization platform in Red Hat Enterprise Linux 6. Other Linux vendors followed Red Hat's lead and started focusing more on KVM as well.
Xen hypervisor makes a comeback
Even as everyone seemed to turn their backs to Xen, Citrix was still using it and developers were still working on it. Some Linux vendors, like Oracle and SUSE, were still offering products based on Xen. Oracle, for instance, offers Oracle VM, a Xen-based server virtualization product.
But things started to change for Xen in 2011, when it was finally officially approved in the Linux kernel. In April of this year, the Xen open source project moved under the auspices the Linux Foundation, with contributors like Amazon, AMD, Cisco, Citrix, Google, Oracle, Samsung and Verizon.
Now that Xen is officially in the Linux kernel and is under the auspices of the Linux Foundation, things are likely to change for it. At this moment, none of the major Linux players have announced plans to support Xen, but it's still early. Some Linux distributions, like CentOS, have already made solutions available that are using the Xen hypervisor, and, given the current contributors to the Xen project, it is likely we'll see more very soon.