xHCI-IOV is an xHCI-specific extension to SR-IOV (Single Root I/O Virtualization), the PCI specification that lets a single PCI device expose multiple functions.
Of the functions, one is a physical function and the others are virtual functions. Virtual functions have some limitations regarding the features they can support, but the spec says that no function cannot interfere with the functioning of the others.
However, I don't think Linux supports it. The XHCI_EXT_CAPS_VIRT
symbol only appears in debug code, and there should probably be a separate driver for virtual xHCI functions as well.