One of the oldest (and hottest) topic in the history of modern virtualization is if the Xen open source hypervisor can be integrated into the Linux kernel or not.
XenSource tried to achieve the goal for years (while VMware did its best to avoid it), but in December 2006 Linus Torvalds announced the decision to include another virtualization platform in place of Xen: KVM.
KVM was developed and maintained by the startup Qumranet, acquired by Red Hat in September 2008, and at that time was just 6 months old, much less mature than Xen.
Despite that and because of its architecture (at least this is the official reason), KVM has been included in the kernel since version 2.6.20 and Xen is not.
Torvalds’ comment on the idea is lapidary:
…If Xen was a single driver thing, we wouldn’t have this discussion. But as is, Xen craps all over OTHER PEOPLES CODE. When those people then aren’t interested in Xen, why is anybody surprised that people aren’t excited?
Thanks to c0t0d0s0 for the news.