ESVM = Enhanced Simplified Voicemail. Very simple voicemail. This is built-in and free on the NS700. I only use if the customer just needs 1 mailbox as an answering machine. There are no audion prompts.
VM = TVA voicemail integration (don't use this on the NS since you can use the UM)
UM = Unified Messaging. This is activated by adding a DSP card and SD card. With those two, you get the UM w/ 2-channels. If you need to use more channels add licenses. UM is a built-in version of the TVA and more advanced. The more I'm installing, the more I like the UM.