This page has a comparison of the various IO assistance modes GVM can make use of (see comparison of assistance modes, the Mdev Mode section, and the SR-IOV Mode section):
https://openmdev.io/index.php/Virtual_IO_Internals
This will probably also play a role in future developments like SIOV (Scalable IO Virtualization):
https://lwn.net/ml/linux-kernel/20190222021927.13132-1-baolu...