Live migration is possible with KVM, too - in combination with DRBD for shared storage, another open-source solution.
Sure, but I was comparing ESXi to VMWare Server.
There are several options if you want live migration, I believe that Citrix XenServer supports live migration in their free version. Maybe even the free version of Microsoft Hyper-V Server can do it too, I'm not sure.
Either way, AFAIK, KVM is not a supported paltform by Microsoft, so it's not viable in an enterprise environment were support contracts are a must.
Cheers, Daniel