SR-IOV – Single Root I/O Virtualization. This is an extension to the PCI Express (PCIe) specification and allows a device, such as a network interface card (NIC)  to present itself as multiple virtual devices to a hypervisor. There is an SR-IOV standard created by the PCI-SIG (Peripheral Component Interconnect – Special Interest Group) with help from Intel, IBM, Hewlett-Packard, Microsoft and others. SR-IOV works only within a single server; that’s what the single root implies. Within a server there is a core PCI root complex interconnecting all the server’s PCIe devices, such as NICs, bridges and switches. Envisage a tree-like structure with them cascading off the root complex.

A corresponding MR-IOV (Multi Root I/O Virtualization) refers to sharing PCIe devices among multiple computers.