What I haven't yet seen mentioned are a couple advantages to it is the ability to manage VirtualBox via alternative frontends like phpvirtualbox, although it annoying requires Java runtime (sorry, not a fan) for remoting in VNC-style.
And as mentioned above VB is portable across Windows/Mac/Linux and unofficially FreeBSD hosts making it easy to move stuff to different OSes like that without any issues. That's the only downside I can see to the KVM/XEN kernel stuff: you're kind of tied to that VM technology on that host OS. But then I may not know what I'm talking about.
--semi-off topic rant
I've been using VB for years now but I started looking at alternatives a little while ago: whilst VB can have an incredible number of virtual hard drives of hundreds of gigabytes each Microsoft's VirtualPC supports only three virtual drives of a max capacity of 160 gigs each. I mean...what the hell...