Windows 8 virtualization gives BSOD 0x0000005D

I find that, when trying to run windows 8 on Xen cloud (and also when running it in ESXi), the installer bluescreens with

0x0000005D (UNSUPPORTED PROCESSOR)

Initially I thought it was giving the VM a 32-bit processor (I was using a 64-bit image), but this was not the case. PAE is enabled. What else could this be?

Additionally, trying to do it on VMWare ESXi allows it to boot, but it remains on the loading screen forever during the first reboot during setup. Why?

It turns out that Xen, by default, keeps the NX CPU feature disabled for VMs; Windows 8 requires this feature. You can change this in Xen cloud using this command:

xe vm-param-set uuid=<your-uuid> "platform:nx=true"

Then, as if by magic, it boots.

On ESX(i), the same results can be accomplished by ensuring the XD bit isn't masked out in the VM configuration, which is the default. However, it is also necessary to set the virtual hardware to use EFI instead of BIOS, which is not the default.

In ESXi 5.1 this is no longer a problem and the BIOS emulation can boot windows 8 by default.