Just a bunch of notes after reading:
VirtualBox == have to export VM then import it at the new destinationto keep it happy. A failed shrink puts the VM into an inconsistent state.
VMWare player (WMWare free?) == Missing a virtual disk management tool. Check for supported hardware? Better with 3D video. Uses more memory than VirtualBox
VMWare free == move copy the folder containing the VM and most of the time it’ll run just fine in it’s new location, no USB?
VMWare workstation == best but expensive. Easily move from one OS to another with just drag and drop (I assume this means XP to Win7/Win8 or Linux)
OpenVZ == not windows
KVM == bare metal, but setup isn’t easy and requires a Linux host. Works well for vituralization of servers (headless).
Hyper-V == Good for Windows VMs. Not useful for Linux VMs.
Xen == not for beginners, “Art of Zen” useful book
ESX ==