[ubuntu] Confused about terminology - what do I need?

I'm not actually sure what to call what I want to do. Is it cloud computing? Or is it OpenStack or what?

I have two VMware clusters of three physical servers each. I am tired of paying annual maintenance fees so I am looking at open source alternatives. I have used Ubuntu servers for 4 or 5 years so I am familiar with them.

Questions:
From what I can see if I use Xen or XCP I have to run Cento as the base OS, but I can run Ubuntu as the guest. Is that correct?

If I want to replace my VMware clusters can it be done with Ubuntu and KVM? Or do I need additional software?

It appears that Ubuntu and KVM only supports GUI-less guests, that would seem to preclude a Windows server guest or can Windows Core run as a guest?

Thanks