...I have no experience that this will work with KVM and ProxMox in particular,
but what you'll need is a motherboard AND a CPU with support for virtual-I/O (i.e Intel vt-d).
Under VMWare ESXi, the feature "vmdirectpath" relies on it as well. There's a HCL available for this product.
I personally have good experience with Supermicro motherboards from the XEON class (like the X8SIL).
My "green" server runs that mobo with a L3426 CPU and vt-d successfully.