Hats off to the Xen developers like Jeremy Fitzhardinge and others that persevered despite what was quite stiff resistance from the Linux community initially! Just to clarify, the major stumbling block was the Dom0 - Xen's privileged domain which has normally been a fairly heavily customized Linux kernel. A number of the DomU/guest components were already in the mainline for a while. Then the core Dom0 components went in around 2.6.37. And now all the PV backend drivers etc. are now in as well, paving the way for a vanilla Linux kernel to be a fully functional dom0 under Xen.

Having watched the back-and-forth between the Xen and Linux community over this inclusion business a bit, I feel the delay has actually been worth it in hindsight. Linux's pvops abstraction is clean and impartial. With this Linux has support for at least 3 virtualization technologies out of the box: KVM, Xen, and lguest. Also, a lot of the jagged edges in the Xen dom0 patches have been smoothed in this process.

This is also great news for private and public Xen-based deployments as well as companies such as Oracle, Suse and others that built their own virtualization platforms on top of Xen. Trying to get supported, well-tested drivers for new hardware working with Xen has often been a major pain point and required significant engineering resources. This was a major reason why Ubuntu switched to KVM officially (Red Hat led the charge here but that was mostly because they bought Qumranet - the company that built KVM). With this Xen is significantly more maintainable and I expect to see at least some of the distros to re-include xen as a virtualization option fairly soon!