No no no not kvm Vmware are the best
Yes and No. Each has their own strengths and their own weaknesses.
I use ESXi in production because the smaller footprint and 3rd party tools I have make management & backup exceptionally simple to use. I'm also supporting a mixed Windows / Linux shop so I already have a Windows PC that runs the management apps. Only thing I don't like is not being able to log into VM's from the console, something I can do with KVM/Xen.
I'm playing around with KVM and Xen at home because the host in each case is my home PCs. I use them because I have a few tools I use as a student that only run on Windows, and my wife isn't fully converted to Linux yet. :-)