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.