SVS

from The Free On-line Dictionary of Computing (8 July 2008)

OS/VS2
Single Virtual Storage
SVS

   <operating system> An {operating system} from {IBM}, released
   with the {IBM S/370} and originally known as {OS/MVT}.  OS/VS2
   R1 was known as SVS (Single Virtual Storage) as it had a
   single 16 MB {virtual address space}.  OS/VS2 R2 was known as
   MVS - {Multiple Virtual Storage}.

   (1999-01-13)