Matt, our good friend, and openQRM Project Manager, wrote about it in NetworkWorld, quoting him :
See the rest of that interesting response!
To my mind it is because of two reasons :
1) Different virtualization technologies are available today. There is "full-virtualization" (e.g. Qemu/KVM), "para-virtualization" (e.g. Xen) and "light-virtualization" which is in most cases based on a process-isolation (e.g. Linux-VServer, Solaris Zones).
Each technology has its advantages but also its "limitations".
Now, in a common data-center we normally find lots of different applications with custom needs e.g. a web hosting company may have hundreds of "idle" customer web-servers plus a couple of in-house oracle data-base servers.
For the web-servers it would make the most sense to choose one of the "light-virtualization" or "para-virtualization" technologies to limit the virtualization overhead to a minimum.
Using "light- or para-virtualization" a single, physical machine can easily host several hundreds virtual partitions.