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NV is an overlay; it's a tunnel. Rather than physically connecting two domains in a network, NV creates a tunnel through the existing network to connect two domains. NV is valuable because it saves administrators from having to physically wire up each new domain connection, especially for virtual machines that get created. This is useful because administrators don't have to change what they have already done. They get a new way to virtualize their infrastructure and make changes on top of an existing infrastructure.
NFV also reduces the need to overprovision: rather than buying big firewall or IDS/IPS boxes that can handle a whole network, the customer can buy functions for the specific tunnels that need them. This reduces initial Capex, but the operational gains are the real advantage. NFV can be thought of as a parallel to VMware, with a few boxes running a lot of virtual servers, and a point and click provisioning system.
While NV and NFV add virtual tunnels and functions to the physical network, SDN changes the physical network, and therefore is really a new externally driven means to provision and manage the network. A use case may involve moving a large "elephant flow" from a 1G port to a 10G port, or aggregation of lot of "mice flows" to one 1G port. SDN is implemented on network switches, rather than x86 servers. BigSwitch and Pica8 are examples of companies selling SDN-related products.