QEMU is a processor emulator, its usefulness in gaming is limited to games that require more processor cores artificially for certain functions, like BF4 to a certain extent, where it's not possible to hack the artificial software limitations out of the game.
KVM by far offer the best performance, it's faster than Xen, so it definitely is a better option for gaming.
I have scripted the setup when I was using it, with a VGA passthrough. But I haven't been using my Windows container for gaming since february 2013, the last time when I played a Windows game in the Windows container. Because of the countless Windows updates since then, the Windows install in that container is now broken, and I haven't bothered fixing it because I have better things to do than to spend my time fixing inherently broken malware, and I don't game a lot, but the games I do play, run in Linux. I've decided to be consequent about software: I don't use Windows software period, and that includes games that don't run in wine. Most of my games do run in wine just fine though.
To configure kvm for VGA passthrough, you have to script the container. For PCI passthrough (dual graphics), it can be done just using virt-manager, but to be honest, you wouldn't know how to configure what without knowing some CLI commands to get information about the system that you need to configure the PCI passthrough (not difficult, just dmesg, lspci, glxinfo, basic stuff like that).
The most important thing to know for VGA passthrough to work, is that it will only work with AMD cards, not with nVidia cards, and that you can only use KMS drivers in your host, because KVM uses the host's kernel, and you unbind the GPU from your host when starting the KVM container. So you can only access your host through ssh once you start up the container, which means that you have to think that through before starting the container, so that you can get to your host, without exposing the container or the host to security risks.
Xen is a viable option, it is only a bit slower than KVM, and it has the benefit of also working in Android (which is still in alpha, so it "kinda" works. The problem with Xen is the distro support. Some distros have a good Xen integration, like OpenSuSE, but most distros just "run" Xen, but do not offer full hypervizor implementation with the same functionality as running Xen Hypervizor on bare metal.
If you have and AMD card, and a dual graphics adapter (AMD or Intel spare GPU for the host when the main AMD GPU is passed through), it's not terribly difficult to set up a PCI passthrough with kvm, but there is no single config, it depends a lot on the specific hardware, so it's just not possible to give a "how to" that will work on all systems. People with nVidia cards or unsupported motherboards, are just out of luck. I've said this before and the193rd also explained it in this thread, but it's not because your CPU has VT-d that you're in the clear: a lot of mobos block the address translation that is necessary to run a passthrough.
As to 3D CAD, check out this "Solidworks-like" 3D CAD program, which is commercial closed source software for linux, but they offer a free trial version to get an idea of what it does: http://www.varicad.com/en/home/products/download/