I have tried both XEN and OPEN VZ virtualisations and now feel to try the KVM..
According to an article I read recently
KVM Virtualization is the best option when you need full OS and kernel control. This is as close as you’ll come to a “real” server as we provide ISO Files for you to mount in your “Virtual CD”. OS Installation is not automated so you have to know how to install your Operating System. The benefit of using an ISO file is that you can load a larger variety of OS’s like Windows, FreeBSD etc.
XenPV is optimized for resource granularity and para-virtual devices which enables a near-native level of speed. OS Installation is automated so it’s just a click to install CentOS, Debian, Ubuntu, Fedora, ArchLinux, Scientific Linux. Due to the one click OS Reinstalls, XenPV is great for experts, developers and beginners alike. Pygrub also enabled for many OS templates.
OpenVZ’s model of Virtualization allows for the sharing of RAM across all VPS servers which allows for higher RAM allotments. Most VPS servers never use their full quota of RAM, OpenVZ takes this RAM and allows others to use it. If this sharing is done correctly, everybody has more RAM to use without adversely affecting their system. Our OpenVZ nodes are 48 Core Servers with at least 100GB of RAM so there is always plenty of CPU cycles and a good-sized free pool of RAM left for sharing.
From my experience since last few years, I love XEN compared to OPEN VZ