Oracle and Amazon were the last holdouts for Xen deployments and they recently decided to drop the hypervisor in favor of KVM.
This means there are less resources channeled into Xen development, hardware support and security reviews.
I think that regardless of views on the hypervisor religious wars, this is another example of the importance of mainlining. The upstream solution always wins out in the end and the alternatives fade into obscurity. See aes-loop vs dmcrypt for example.
Hear the screaming and bone crunching nearby? That’s VMware’s ass being steamrolled by KVM.