Well, you can always make a distro leaner, by installing less, and only the essential packages.
It also depends on what type of VMs you're planning to run. Most VMs I run, is on Fedora, but that might not be what you need, if you plan on using XEN-based VMs.
When in doubt, I'd advise testing several distros over a period of maybe two or three weeks, and giving it some performance and security testing.
You might also want a more hybrid approach, and regarding the cluster of hosts and guests as a VM realm.
If security is a big issue, personally, I'd use an OpenBSD firewall (with
pf) managing access. If you need many different VM solutions, for instance some running on VMWare some with KVM, some XEN or VirtualBox, I'd choose what's best, even if that means having different VM solutions running side by side on several machines.
I suggest you take a distro, that's easy to manage at first, for instance Debian, many packages, but you can install a minimal system. This will make it easier dealing with drivers, as well as giving you the selectability, so you can trim the system down for maximum performance.