If you are doing a lot of Linux on Linux virtualization, check out the two OS Virtualization
systems available for Linux: 1) OpenVZ and 2) Linux-VServer. Both are good but my personal
preference is OpenVZ.
Xen and KVM work well... but... OS Virtualization is much lighter and more efficient than
machine / hardware virtualization and provides much greater density and scalability. The
large number of dynamic resource management parameters offered by OS Virtualization are a big
bonus too.