KVM is the future king
There is little doubt that KVM will be the king of hypervisors. Xen was a temporary solution, and it worked well enough, but KVM takes full advantage of the facilities already available in the Linux kernel. It is truly a part of the kernel, whereas Xen was merely co-resident with it.
KVM is blessed by Red Hat, Canonical, and some dude named Linus Torvalds, maybe you've heard of him :)
VMware's current awesomeness stems from its management tools. vSphere is a pretty sophisticated piece of software. It has its warts but there's currently nothing like it. I'm psyched to hear about the Open Virtualization Alliance because it means KVM will gain similar tools. And I'm looking forward to deploying KVM in bigger and bigger environments.
Microsoft? Hah. Non-starter. Just like in mobile -- nobody wants what Microsoft is selling.