In the Fraser article there are a lot of interesting informations like a comment from Zachary Amsden:
Zach was asked “Why can’t vmware use the Xen interface instead?” and he responded:
We could. But it is our opinion that the Xen interface is unnecessarily complicated, without a clean separation between the layer of interaction with the hypervisor and the kernel proper. The interface we propose we believe is more powerful, and more conducive to performance optimizations while providing significant advantages – most specifically, a single binary image that is properly virtualizable on multiple hypervisors and capable of running on native hardware.
Read it at source.