Wow!
I just found Proxmox Virtual Environment (v1.1) which is kind of combination between VMware and OpenVZ.
There is the possibility to run Virtual “chrooted” Appliances and “real” Virtual Machines.
That means, you have your Linux boxes (OpenVZ style) and Windows machines (KVM virtualization).
-> (Almost) NO overhead, when running linux clients (because of chrooted environment)
-> Possibility to run Windows machines, etc., too

Features:
– Out-of-the-box installation (insert CD, [Enter], [Enter], ready)
– Web based management
– Cluster functionality
– Live migration
– Install clients from ISO files (full virtualization)
– Run (OpenVZ) templates (container virtualization)
– Backup / Restore (via LVM snapshots)
– Testing Proxmox VE in VMware
– ….

I think it’s worth to have a look at …

Note: 64bit CPU required; for full (KVM) virtualization, you need Intel VT / AMD-V support.