Prerequisites
A VGA Passthrough relies on a number of technologies that are not ubiquitous as of today and might not be available on your hardware. You will not be able to do this on your machine unless the following requirements are met :
Your CPU must support hardware virtualization (for kvm) and IOMMU (for the passthrough itself)
List of compatible Intel CPUs (Intel VT-x and Intel VT-d)
All AMD CPUs from the Bulldozer generation and up (including Zen) should be compatible.
CPUs from the K10 generation (2007) do not have an IOMMU, so you need to have a motherboard with a 890FX or 990FX chipset to make it work, as those have their own IOMMU.
Your motherboard must also support IOMMU
Both the chipset and the BIOS must support it. It is not always easy to tell at a glance whether or not this is the case, but there is a fairly comprehensive list on the matter on the Xen wiki as well as Wikipedia:List of IOMMU-supporting hardware.
Your guest GPU ROM must support UEFI.
If you can find any ROM in this list that applies to your specific GPU and is said to support UEFI, you are generally in the clear. All GPUs from 2012 and later should support this, as Microsoft made UEFI a requirement for devices to be marketed as compatible with Windows 8.
You will probably want to have a spare monitor or one with multiple input ports connected to different GPUs (the passthrough GPU will not display anything if there is no screen plugged in and using a VNC or Spice connection will not help your performance), as well as a mouse and a keyboard you can pass to your virtual machine. If anything goes wrong, you will at least have a way to control your host machine this way.
