It's been a while since I compiled a Linux kernel from scratch. I was upgrading a Gentoo VM today and was reconfiguring the new kernel and saw that, not only is VMI PV support built in to the standard kernel, but so is Xen PV support. And, you can compile both in at the same time. Combine that with Xen's support for VMDK files, and it looks like I now have the possibility of creating PV virtual machines and moving them back and forth between my Xen hosts and my ESX/ESXi hosts. If I use an NFS share, I may be able to have the same set of VMs accessible in both places at the same time.