Simon Crosby is a dink.
He makes Xen look bad by spewing XenSource marketing crap all over the place.
FWIW, Keith, I think, is assuming that we switch the full page table out to provide isolated userspace/kernelspace memory environments. We don't do this. We simply bring in a PGD for kernelspace on switch. This doesn't invalidate the whole TLB so it really isn't that bad. The problem is that the kernel has to run in ring 3 to ensure that it cannot get at the hypervisor's memory because segmentation was removed from x86-64.
Update: I should point out that Simon is a nice guy overall. I'm somewhat bothered because I'm afraid that Xen does not get the respect it ought to get because there is so much FUD surrounding it in the press. Everytime someone claims that Xen is the only robust, enterprise-ready, virtualization solution we lose a little more credibility. The truth of the matter is that Xen is a young Open Source project with a lot of promise. It's not nearly as robust as other more mature projects (like the Linux kernel) and it's certainly not as robust as enterprise hypervisors like IBM's PHYP. That's not to say that it won't be in time but I think we would do better as a project if everyone involved in the project was a bit more forthright.