If features and costs (free) were identical across the board, I would choose KVM hands down.
I love being able to run off Fedora Server, plus all the doors that open up by doing that... which you can't get from Hyper-V or VMWare.
Sure Xen can be installed on there too, but it's dieing and I'm less familiar with it.
I've always liked a tiny hypervisor and push the management off to API's (That can have layered UI/CLI) rather than install the damn kitchen sink on the hypervisor. What value does Fedora Server bring for actually running on the KVM hosts? You need to run Containers on ring 0 or something weird?
Red Hat has been looking at running their OpenStack platform in OpenShift on RHEL Atomic. Not as small as ESXi but it’s around 700MB.
That way nothing is installed in the OS at all. You can actually rebase between Fedora and CentOS/RHEL in Atomic and it doesn’t touch any of your apps.