I'm looking to test some of the new features in Hyper-V 2012 (v3).
Hyper-V v3 can be downloaded as a "free" version "Hyper-V Server 2012", or it can be purchased as part of Windows Server 2012 Standard or datacenter. However, as usual licensing is unclear.
On the one hand MS talk about their free edition in several (many) sites. On the other hand, when you go to the actual download site it talks about a trial. To me a trial has an expiry date so it makes me nervous. I could use my Windows Standard 2012 license as part of my Microsoft Action Pack Subscription (MAPS), but I'm not sure what I'm actually entitled to.
So my question is: What are the real differences between the free (trial) download of Hyper-V Server 2012 and the paid-for Windows Server Core 2012 where you have to install with a key. Does the "trial" version actually expire?
No, Hyper-V Server 2012 doesn't expire.
Hyper-V Server is quite a bit like Server Standard Core with all of the roles except Hyper-V (and other supporting roles and features) removed.
Now, in Server 2012, you can add the full UI back to the Server Core editions, but that's not an option with Hyper-V Server 2012 - it will always just be a command-line. That also means that the typical management UI tools won't run on Hyper-V Server 2012, so you'll need a machine that you can manage it from remotely (the PowerShell cmdlets for Hyper-V actually do work on Hyper-V Server, though).
Hyper-V Server isn't really for people who want to "play around" with Hyper-V - it's really designed for people who want to boost their Hyper-V infrastructure with more physical hosts, and who want to run a very lightweight OS in the root partition, leaving the most resources available for the VMs.
If you just want to get used to Hyper-V or test some things out with it, but you don't have experience with managing Hyper-V remotely already, stick with a full version of Windows Server (or Windows 8 Pro/Enterprise x64, which also have Hyper-V).