Prgmr.com uses the Xen hypervisor, our current servers all use Xen 3.1 (the one that comes with CentOS 5.2 ) This means that all Linux kernels for Xen3.0-3.3 that are running the xen.org Xen patchsets should work just fine.
NetBSD 5-beta works on the current prgmr.com systems and will continue to work on the Xen 3.3 servers.
[FreeBSD] Xen support is in its infancy, but the stuff in 8-CURRENT only works on Xen hypervisor version 3.2 and above, so this will not work on the current prgmr.com servers, but will work with the newer prgmr.com servers running Xen 3.3
Centos 5.x, Debian 4.0, Ubuntu 8.04 (but not 8.10) are known to run on prgmr.com systems. Ubuntu 8.10 and fc10 will work if you use an older kernel, and they will work with distro kernels on the new servers once prgmr.com has Xen 3.3 servers up and running.