Herbert,
NV probably means "Niederverdichtung", low compression ratio. Engines with lower compression ratio were available for countries with lower octane gasoline, to prevent knocking.
Engines with NV had a different "Baumuster", i.e. engine number. E.g. a 280SE engine number with regular compression ratio might start with 130.980...., while the low compression version would start with 130.984....
Klaus