64bit VM on Windows 7 x64

I assume you've read through the official VirtualBox documentation on setting up headless systems, right?

http://www.virtualbox.org/manual/UserManual.html#vboxheadless

Did those instructions not work for you? They specifically mention 2 different ways to start a headless VM. Have you asked on the VirtualBox forums?

I would say it isn't possible to use Hyper-V on Windows 7. You could put Server 2008 with Hyper-V as the host and use Windows 7 as a guest machine. Your best bet is to either use the new Windows Virtual PC or VirtualBox and the like.