Single-Root I/O Virtualisation is an enhancement to PCI passthrough that allows for multiple VNFs to share the same physical NIC.
SR-IOV will emulate multiple PCIe devices on a single PCIe device such as a physical NIC.
In SR-IOV the emulated PCIe devices are called virtual functions, the physical PCIe devices are called physical functions.
The VNFs have direct access to the virtual functions, using PCI passthrough technologies.
The SR-IOV enabled physical NIC supports two different modes for switching traffic between VNFs:
Virtual Ethernet Bridge (VEB): Traffic between VNFs attached to the same pNIC is hardware switched directly by the physical NIC
Virtual Ethernet Port Aggregator (VEPA): Traffic between VNFs attached to the same physical NIC is switched by an external switch