Just quick summary.
NPV is CISCO term doing the same thing like Brocade Access Gateway or DELL Force10 NPG (NPIV Proxy Mode). All these technologies put the Fibre Channel switch in to the mode where they don't have Fibre Channel Domain ID and therefore works like absolutelly transparent Fibre Channel multiplexer or intelligent pass-through if you wish. It significantly simplified SAN architectures and multivendor interoperability.
NPIV is the feature allowing Fibre Channel switch operates more FCIDs over single fibre channel switch port. So it effectively allows aggregation of more Fibre Channel Nodes (N-Port IDs) per single FC link.