As I wrap up my contributions to UF's, or more importantly Renato Figueiredo's, Virtual Computing course, I am putting together a Debian Xen image, which should work in Simics and a host of other VMMs. Because VMware is the least compatible, I always build my initial image in VMware and then quickly continue working on it in a chroot environment of course after converting it into a raw disk format. Anyway, in my most recent attempt, I was trying to figure out why Xen would not boot inside KVM, when it worked fine in VMware and Qemu.
First a little background, Xen was compiled for 32-bit and running a 32-bit userspace, while my KVM install was 64-bit. As you may or may not be aware, x86-64 does not support privileged rings any more and instead relies on the paging modes (U/S) for privilege. Xen 32-bit relies on the rings for performance purposes. So you would think that KVM would support the rings as well. As it turns out, KVM 64-bit does not appear to virtualize the rings and thus Xen 32-bit does not work.
While this is still somewhat of a hunch, I tested a Xen 64-bit, which does not use the privilege rings. In KVM 64-bit, it worked flawlessly. Leaving me two more things to check: determine if Xen 32-bit boots in KVM 32-bit and if a simple ring test boot code works in 32 or 64 bit KVM. The former would be useful for other's, while the latter would be a conclusive simple test to verify the functionality.
Anyway, the point is, you can run Xen in KVM, if they are compiled for the same platform. I'd be curious to see if you can run a 64-bit VM in 32-bit KVM and if so, if the Xen 64-bit would work.