Most likely you’ve all seen this post by Hans Vredevoort on how to create a bootable VHDX in order to test WS 2012 R2/Windows 8.1 Preview without jeopardizing your current installation.
If not, please read it – it’s the most painless method by far.
Another small note; if you are using another base language on your RT device than the officially supported 13 languages you will get an error when trying to install the 8.1 Preview. Please see this post for more info.
Hyper-V according to me 