The file extension SAV is generally used for so-called savegames, that is, games of various computer games. Sav does not represent its own format. Sav files can only be reloaded with the game with which they were saved. Often the version numbers or patch versions can not be different.
The content of a SAV file is of course different from game to game, but always includes information such as player's position, items, and progress, and often much more. The file extension SAV is used among others by Half-Life 2, Mount & Blade, Spellforce 2, OpenTTD and Dune.
For an unknown SAV file, it may be helpful to load it into a hex editor. Sometimes you can tell in the first lines to which game it belongs, if it is a savegame file.