A: System Center Virtual Machine Manager (SCVMM) 2012 manages the three major hypervisors: Microsoft Hyper-V, VMware ESX, and Citrix XenServer.
For ESX management, an instance of VMware's vCenter must be deployed. This is because ESX doesn't expose APIs to allow SCVMM to directly interface and perform management, so all management is performed via the vCenter instance.
Although XenServer has XenCenter, which is its equivalent of VMware's vCenter, SCVMM doesn't have to communicate to XenServer via a XenCenter instance. Instead, it communicates directly to the XenServer after you download and install a SCVMM 2012 add-on for XenServer, which is available from Citrix. This add-on acts like the SCVMM agent that's deployed to Hyper-V servers.