Well SPICE is providing the link between the guest OS and the client machine for display interaction purposes. It has the ability to tunnel access to limited devices, in particular smartcards and USB devices attached to the client machine.
PCI passthrough though is a different scenario - it is enabling the guest to use hardware resources present on the physical host. The client viewer machine isn't involved in this at all. Assigning dedicated virtual functions to each guest, from a SRIOV NIC on the host is most common use case for PCI assignment. VGA passthrough is a less common and more limited use case, since there aren't any multi-function VGA devices you can only help 1 single guest per physical host device.
The two I see on a regular basis is Storage and Network through SRIOV. The other one that is coming up in discussions more and more is GPGPU resources. I know amazon offers this but TBH I'm ignorant as to how they provide this functionality, whether the device is provided by pass-through or whether it's accessed in some other means.
Peter