OK! This took some time to figure out! But its really simple now that I got it figured out!
First you have to extract the GNS3.IOU.VM.ova. OVA is just a compressed archive of 2 files. A VMDK and an OVF (Similar to an XML its the settings for VirtualBox, but we don't need that just the VMDK)
(You will get an Error, thats fine!)
Then, in this example I place GNS3 IOU VM-disk1.vmdk in C:\tmp.
Open CMD.exe in Admin mode and run the follow 2 commands [This Command requires VirtualBox to be installed _ VBox can be installed beside Hyper-V]
vboxmanage clonehd "C:\tmp\GNS3 IOU VM-disk1.vmdk" "C:\tmp\GNS3 IOU VM-disk1.vhd" -format VHD
Now you have your VHD file which is the format Hyper-V accepts.
Its time to create the New VM in Hyper-V! But first you need to have an "Internal Hyper-V Switch"
So follow along if you don't have one.
Right click your computer name under Hyper-V Manager >> Virtual Switch Manager
Select Internal >> OK >> Change Name (I use Hyper-V Internal) >> Apply.
Now create VM via right click your computer name under Hyper-V Manager >> New >> Virtual Machine
Name : Whatever you Want -- Cisco-IOU-VM
Generation: 1 --MUST BE 1
Startup Memory: Not sure the Minimum, but I used 1GB w/ Dynamic
Configure Networking: Connection: (From Earlier) "Hyper-V Internal" *Or whatever you named it*
Connect Virtual Hard Disk: Use an Existing Virtual Hard Disk >> Point to VHD we created earlier
Installation Options: Install an operating system later
Now fire up the VM via right click on VM >> Start && right click VM >> Connect
We need to change the Network Interface to static IP. On the VirtualBox it just auto sets up to 192.168.56.101, so we will use that!
check IP config via ifconfig >> shouldn't have an IPv4 address yet.
Make it look like it is below
save via (CntrL+O >> Enter >> Cntrl+X) then reboot via "reboot"
check IP config via ifconfig >> should say 192.168.56.101
Ping from Host via CMD.exe >> should get a reply!
Visit in browser 192.168.56.101:8000. If you get the screen with GNS3 Server, then continue on to the guide linked at top for actual adding the Images etc etc.