There are multiple methods for virtual machine graphics display which yield greatly accelerated or near bare metal performance.
QXL video driver and SPICE client for display
QXL/SPICE is a high-performance display method. However, it is not designed to offer near-bare metal performance.
PCI GPU passthrough
PCI VGA/GPU passthrough via OVMF
PCI passthrough currently seems to be the most popular method for optimal performance. This forum thread (now closed, and may be outdated) may be of interest for problem solving. You can use kvm switch to control desktops.
Single GPU passthrough
Currently, PCI passthrough works for dual-graphic cards only. However, there is a workaround for passing a single graphic card. The problem with this approach is that you have to detach the graphics card from the host and use ssh to control the host from the guest.
When you start the virtual machine, all your GUI apps will be force terminated. However, as a workaround, you can use Xpra to detach to another Display before starting the virtual machine and reattach the Apps to display after shutting down the virtual machine.
There is a fairly recent passthrough method called Looking Glass. See this guide to getting started which provides some problem solving and user support. Looking Glass uses DXGI (MS DirectX Graphics Infrastructure) to pass complete frames captured from the virtual machine's passed-through video card via shared memory to the host system where they are read (scraped) by a display client running on the bare-metal host.
LibVF.IO is a Virtualization Framework (Libvirt's alternative) for simplifying the GPU Virtualization. It supports Intel (Intel GVT-g, SR-IOV), NVIDIA (NVIDIA vGPU, SR-IOV) and AMD (AMD SR-IOV). You have to create YAML configurations for each virtual machine. Currently, Intel and NVIDIA GPUs are tested, with limited support for AMD. You can follow this setup guide. You can also check their Wiki. For NVIDIA GPU, you need to unlock vGPU which can be done by installing AUR or building it yourself and putting it in LIBVF.IO's Optional Folder.
There is also LIME (LIME Is Mediated Emulation) for executing Windows programs in Linux.
This framework was tested for gaming. By default, LibVF.IO uses Looking Glass as Virtual Display but you can change that through YAML configuration.
Follow this guide to manually setup a Windows 10 guest with NVIDIA vGPU.
Single Root I/O Virtualization is under development by Intel and NVIDIA New GPU Series. There are some AMD GPUs that support this technology such as the W7100.
Intel GPUs based on Xe architecture and newer also support SR-IOV. However, the mainline Linux kernel does not yet support the feature, and you will have to install a custom kernel from Intel. There is also an i915 DKMS kernel module AUR to simplify the process.
- For Linux guests you will have to have the supporting i915 module installed using either the intel-lts kernel or i915-sriov-dkms-git.
- For Windows guests you must install the latest drivers and set the hypervisor's vendor ID to
Intel-specific iGVT-g extension
iGVT-g is limited to integrated Intel graphics on past Intel CPUs (starting from Broadwell and ending with Comet Lake). This is a "software workaround" for older iGPUs that do not support SR-IOV . Newer Intel iGPUs can use SR-IOV instead. For more information, see Intel GVT-g.
Virgil3d virtio-gpu paravirtualized device driver
 virtio-gpu is a paravirtualized 3d accelerated graphics driver, similar to non-graphics virtio drivers (see virtio driver information and virtio Windows guest drivers). For Linux guests, virtio-gpu is fairly mature, having been available since Linux kernel version 4.4 and QEMU version 2.6. See this Reddit Arch thread and Gerd Hoffmann's blog for using this with libvirt and spice.
For Windows guests, there's currently an active work in progress on a fully functional OpenGL and Direct3D10 driver on the main GitHub repo's pull request. There's also a report that Red Hat also previously worked on the OpenGL driver, including the project summary but abandoned it soon after. Other drivers include the DOD (Windows kernel) driver and the ICD (Windows userland) driver are available. In addition, see this Phoronix article and its comments.