In case you wonder, as I did a few days ago, what Hyper V is, here is a simplified answer (the Wikipedia will tell you more):

Hyper V is kind of like Virtual PC for Servers on Steroids: it allows you to efficiently run and manage a whole bunch of different VMs on one machine. Hyper V is a role that is added to Windows 2008 Server (or Windows 2008 Server R2). Once you add the role, some strange things actually happen to your Windows Server installation: it actually becomes it’s own ‘parent partition’ on the server, so that it is also virtualized along with the VMs that you create.

There are many differences between Hyper V and Microsoft Virtual PC. First of all, the VMs have access to way more virtual hardware: multiple processors (Virtual PC only gives you one processor, even on a many core machine), and much more memory. It also requires Hardware Virtualization Assistance, which is present on most modern manycore processors, but if often turned off in the BIOS (I had to turn it on on my Alienware M17X, see this helpful tip). Hyper V also comes with all kinds of nifty management tools, and is trying to get into the datacenter-level virtualized server market.

To confuse things further, there are lots of different SKUs for Hyper V. You can purchase Microsoft Windows Server 2008 (and R2), and enable the Hyper V role, or you can get the free Microsoft Windows Hyper V Server 2008… it’s almost like SKU palindromes! The latter is a stripped down version of the full server SKU that is just specialized in the Hyper V role, and preconfigured for it.

Why would a developer care about all of this? Well, VMs are a great way to test new software, for instance I will be talking about Visual Studio 2010 and the .NET Parallel Computing initiative, and the current beta happens to not be uninstallable (sorry about the double negations). So if I install it on my machine I have to reinstall everything for the next beta refresh. Having better things to do with my time (like blogging about Hyper V on Sunday evening), I would rather put it in a VM. If you have been paying attention you will recall that MS Virtual PC only supports one processor, so that isn’t going to do me much good testing and demoing Parallel Processing…. hence the Hyper V approach. In a later blog I will detail my whole setup and post more on my findings preparing for the Parallel Processing talk at Silicon Valley Code Camp 09.