I know it’s possible because sleep works in HyperV under Windows 8.
edit: I know this is probably not “supported”.
On the server version of Windows, you can’t. Once the Hyper-V role loads, hibernate and sleep are disabled.
You can sleep on Windows 8 Hyper-V because it is “Client Hyper-V” in which sleep states remain enabled. (Windows IT Pro has a list of differences between Server and Client Hyper-V, and this appears among them.)
If you really need Hyper-V installed but still want to enable sleep/hibernate selectively, a workaround is available which will disable the Hyper-V role from loading. When it’s not loaded, you can put the machine to sleep/hibernate it. You need to reboot after applying these registry keys.
Gain hibernation/sleep – lose Hyper-V:
Lose hibernation/sleep – gain Hyper-V:
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