KVM for the Mainframe
The s390 is the grand-daddy of virtualization. Everything started there. In so many ways, everything we're doing with x86 virtualization is just playing catch-up. The new exciting features like hardware virtualization support and hardware paging support have been in s390 forever.
s390 clearly has a very mature hypervisor. What many people may not know though is that it's normal to run two hypervisors at any given time on s390. At the bottom level, there's PR/SM which divides the machine into rather coarse partitions. Within a PR/SM partition, you can run z/OS or Linux. You can also run z/VM within a PR/SM partition. z/VM is another hypervisor that allows for much more sophisticated features like memory overcommit and processor overcommit. The user has the ability to decide how much hypervisor they need to maximize the efficiency of their workloads.
Within a z/VM partition, you can run z/OS or Linux. The beauty of s390 is that this configuration has been supported in the hardware for many years and is very fast.
When Linux adopted native support for virtualization, it became obvious that this could be easily supported on the s390. The hardware has long supported this sort of nested virtualization and the implementation turned out to be very straight forward. It helps that the x86 virtualization extensions were inspired by a paper written about s370 almost 30 years ago :-)
What do you get from a platform that has supported virtualization for longer than I've been alive? In this very first release of KVM for s390, it already supports 64-way guests. After two years of development, we've just gotten to supporting 16-way guests on x86.