Hardware pass-through means that the guest VM has direct access to the physical hardware. The hypervisor literally lets it “pass through” to the hardware. Hardware pass-through is the best possible performance for the guest VM since it’s essentially the same speed as if there was no hypervisor at all, but you lose VM portability across hardware of different types.
Hardware Virtualization Support for Networking
The SR-IOV mechanism is an extension of PCI that allows virtual machines to safely access their "slice" of an adapter with efficient hardware operations such as DMA. It is especially popular (and useful) for network adapters, and implemented by many modern Gigabit Ethernet and 10GE adapters.