When you setup a Windows guest on KVM/QEMU you can choose between different video devices, most used seems to be QXL, VirtIO or VGA.
Which give the best performance when you connect to the guest using RDP?
1 Answer
After some thinking, it occurred to me that it will likely not matter at all as the selected video device will not be used, instead you will use the virtual RDP device called "Microsoft Remote Display Adapter"!
So you might as well choose are setup which use as little resources as possible. If you use libvirt XML config file:
<video> <model type='vga' vram='4096' heads='1' primary='yes'/> </video> But to consider:
If you connect to the guest using e.g. VNC or Spice, it will matter though. In my experience QXL doesn't work well with VNC quality-wise, you'll get color and/or image errors, buy QXL is good for spice.
And there are RDP configurations in the local policy on the Windows guest you can make which will make use of the QEMU video device. 
But since QEMU devices (e.g. QXL,VirIO or VGA) is already/also doing software rendering, I don't think it would benefit you....I assume the virtual MS RDP device is optimized for RDP and do the best job. Unless you use passthrough a real GPU device to your guest, e.g. an AMD og Nvidia, then you will benefit greatly.
reddit.com/r/Proxmox/comments/1auvdlgmay be useful.