virtualbox.org

Hello,
I have Windows 8.1 Pro, I installed the latest version of virtualbox (4.3.
I want to install Windows 2012 server in virtualbox and use HyperV.
I installed HyperV on Windows Pro 8.1 physical machine.
When I tried to configure a new Windows Server 2012 machines in virtualbox, I realized that I could not benefit from the VT-x technology (processor tab is grayed out).
How to solve this problem?

Thanks you

Nested virtualization (running a virtualizer inside a guest) is not supported in Virtualbox.

Like Martin said, if you have a 8.1 host and you're trying to run Server 2012 in a Virtualbox guest, then activate hyper-V in the 2012 guest to run a hyper-V guest inside the 2012, that likely will not work. Hyper-V needs VT-x support from its hardware, if I remember right, and Virtualbox doesn't simulate VT-x in its guests.
You did also say that you activated hyper-V on the 8.1 host. If you're trying to run a Virtualbox guest and a Hyper-V guest at the same time on the 8.1 host, you may run into at least some limitations, if not fail to get either or both guests to run. Hyper-V on the host likes to take VT-x when the host OS boots, and won't let Virtualbox use it. So Virtualbox will be limited to 32-bit one-processor guests if it works at all. For VB to run 64-bit guests or multi-processor guests, Hyper-V needs to not be installed. It may come down to one or the other.