Almost definitely KVM. The Ubuntu/KVM/OpenStack combo is the bread-and-butter deployment scenario outside of a small number of folks (notably Rackspace, which is OpenStack on XenServer^TM).

Also, <= Diablo and to a large extent Essex releases were known to be (politely speaking) less-than-usable OOTB. Folsom was really the first release that could properly be called usable for a production IaaS, and Grizzly is even better.

Source: knowing these things is my job.