Multiple Virtual Storage, more commonly called MVS, was the most commonly used operating system on the System/370 and System/390 IBM mainframe computers. It was developed by IBM, but is unrelated to IBM's other mainframe operating system, VM.

First released in 1974, MVS had been renamed multiple times, first to MVS/XA (eXtended Architecture), next to MVS/ESA (Enterprise Systems Architecture), then to OS/390 (when UNIX System Services (USS) were added), and finally to z/OS (when 64-bit support was added with the zSeries models). Its core remains fundamentally the same operating system. By design, programs written for MVS can still run on z/OS without modification.

At first IBM described MVS as simply a new release of OS/VS2. But it was in fact a complete re-write - previous OS/VS2 releases were upgrades of OS/MVT and, like MVT, were mainly written in Assembler; the core of MVS was almost entirely written in PL/S. IBM's use of "OS/VS2" emphasized upwards compatibility: application programs which ran under MVT did not even need to be re-compiled in order to run under MVS; the same Job Control Language files could be used unchanged; the utilities and other non-core facilities like TSO ran unchanged. But users almost unanimously called the new system MVS from the start, and IBM followed their lead in the naming of later major versions such as MVS/XA. After the release of MVS, users described earlier OS/VS2 releases as SVS (Single Virtual Storage).

Source: Wikipedia