Microsoft Hyper-V is the hypervisor. Its latest release is Hyper-V R2 . It supports 384 running VMs and 512 virtual processors. It now support Live Migration and cluster Shared Volumes. It must be noted that it is not free as commoners know . You have to pay for windows server 2008 to run it.
It does not support any other architecture but x86_64 but can run 32 bits guest also. It used hardware assistance like Intel VT or AMD-V. There's some talk of supporting the Red hat offically. Suse is supported. It don't support usb devices for guest.
Hyper-V will not support linux of futures having kernel 2.6.35 as kernel team have removed it due to non submission of codes by Microsoft. Graphics performance is bad. Also no OS support like FreeBSD , netBSD and other linux except Microsoft OS.It support 10 gbps networks. It is a type 1.( though some question about it.
It is not cheaper than other solutions and more established players like vmware and citrix.