BigV has been running since 2011, it's only months younger, but built on expertise from our old VM platform that's 10 years older.
Xen and KVM both drew from the same code, i.e. qemu, but qemu now has all the important patches from KVM, and has featured paravirtualised I/O for years.
So I don't think the choice or virtualiser is fundamental to how you build a hosting platform any more - we could have built BigV on top of Hyper-V if we were masochistic enough ;-) and it would look the same from the outside.
One main difference between our platforms is that our storage is decoupled (but not very far) from the CPU, so you can attach up to 8 discs of different grades to your virtual machine. We can also live-migrate running machines, and running discs to keep things running, rather than just carving up individual boxes, discs & all, in our old VPS model.