The evolution of the data center is clearly moving towards a self-service (Cloud) model. But talking about which hypervisor is correct for the data center is the wrong discussion point to take.
I would suggest that you not think about which Hypervisor is correct for you; rather, you think about which Cloud OS is right for your data center. In this way, you move the conversation and ultimately the strategic direction of your data center up the stack.
So, looking at the Cloud OS and how it relates to the hypervisor, you really have three choices (VMware, Microsoft, OpenStack).
If you are a Microsoft shop (many of your applications run on or will be migrating to Server 2012), then naturally you have expertise around Hyper-V. It might not make sense to move to a different Cloud OS. Why you are a complete Microsoft shop to begin with is a whole other discussion.
If you are a mixed Linux and Microsoft shop, then most likely you are already a VMware shop (good or bad: they own the market). In this scenario, you may be looking to maintain your expertise in VMware (vSphere – ESXi) or potentially looking to deploy a more cost effective Cloud OS for the future. If that is the case, your best consideration is OpenStack.
Within OpenStack, you find major support for 4 hypervisors (XEN, KVM, ESXi, and Hyper-V) with XEN and KVM having the highest integration to date per OpenStack. OpenStack is to Cloud OS what LINUX was to UNIX; open standard community supported Cloud OS that has the backing and implementation of many major hypervisor and global 100 companies.
Obviously there are other hypervisors (IBM, Oracle, etc.) however I would look to the vision of your datacenter as a private cloud, determine which OS will power the cloud, then choose the hypervisor(s) that fits that vision.