The issue of virtual compatibility

Both hypervisors we tested have requirements for the hardware they can run on and the virtual machines they can support.

They both require a supported hardware platform with a 64-bit virtualization-enabled Intel or Advanced Micro Devices CPU. Sufficient memory is needed to support the guests that will inhabit the virtualized atmosphere. VMware's ESX displaces less than half a gigabyte of memory for its own use. Hyper-V must live on (some will argue adjacent to) an edition of Microsoft Windows Server 2008 (the choice of edition decides the number of guests and requisite costs of hosting) but takes up a nominal amount of extra operating-system space. Microsoft's recommended base memory requirement is 2GB -- but includes room for Hyper-V and a Windows Server 2008 base instance.

Hardware choices can be complex because both server and peripheral cards (generally network interface cards, and a disk/host bus adapters infrastructure) need to be supported by the hypervisors as well. Hyper-V runs over any platform that suits Windows Server 2008 editions -- a very long list. The Windows Server site lists approved hardware and software and outlines how to use Hyper-V on top of Windows Server 2008 running in 64-bit mode atop a V- or VT-enabled CPU.
In contrast, VMware's ESX compatibility list includes many servers from the top-tier equipment vendors -- IBM, Dell and HP -- but overall the list is far shorter than that for Hyper-V.

General, white-box, 64-bit AMD and Intel machines are not supported officially by either virtualization platform. If they are equipped with the right virtualized processors and BIOSs, they might work, but support for the problems found in these hosts might not be forthcoming.

Knowing the infrastructure and administrative ins and outs of Windows Server 2008 editions is the ticket to a simple and fast installation of Hyper-V because it runs as a server role snap-in. No initial Hyper-V configuration is required if Windows Server 2008 is installed already. By contrast, VMware's ESX installs like a typical Linux distribution but with a graphical front end.

Both hypervisors were easy to install on our platforms, which were known to be compatible with their product families.
The list of operating systems that can be migrated to each platform stands squarely in favor of VMware's ESX.
ESX’s advantage comes in part from the fact that it supports many versions of Windows operating systems — more than Hyper-V, in fact — ranging from user operating systems (Windows XP and Vista Professional in x86 or x64 versions) to Windows Server operating-system flavors (from Windows 2000 through Windows 2003 x86 or x64 versions to the latest cuts of Windows Server 2008 Data Center and High Performance Cluster versions). It also supports Windows NT.

The other reason for ESX's edge here is that Hyper-V -- as Microsoft certifies -- supports only one version of Linux, Novell's SUSE Linux Enterprise (SLES) 10 Service Pack 1 or 2, in x86 and x64 versions. However, only one virtual processor is supported for each virtualized instance of SLES 10 SP 1 or 2. Microsoft's Connectix acquisition, which brought Microsoft Virtual Server to market, initially supported a vastly wider variety of guests. For Hyper-V support of Linux, Microsoft's relationship with Novell has Microsoft buying hundreds of thousands of SUSE Linux support kits for Microsoft's (and their customers') use. (Listen to a podcast of Microsoft's partnership with Novell.)