Re: Please be more critical of the Qubes project
It's a persistent misconception because it's true. KVM cannot be run without Linux. Xen can, and is, run without Linux. There's the well known kernels (Linux, Solaris and derivatives, NetBSD), the in progress (FreeBSD) and the custom (MiniOS, shipped as an example with Xen, and third party options).
I've just checked the documentation for RHEV, and it's clearly a stripped down Linux. At one point KVM.ko did load under FreeBSD, but that's only because of FreeBSD's Linux compatibility, and even then it didn't work well.
Personally I think of KVM as an optional Qemu accelerator, coupled with some excellent pci passthrough tools. Xen's paravirtualised architecture means it can be quite different, and it's (in my opinion), considerably better integrated than KVM, plus has a (now free) turnkey system using a custom Linux distribution, looking not dissimilar to ESXi.
Obviously there's also the cost, and supportability issues. VMWare is excellent, but will cost a lot in some configurations, has a limited set of supported hardware, and most irritating keeps dropping support for perfectly functional hardware in new releases. At least with Xen/KVM, it's using standard OS drivers.