What is the difference between V$ and GV$, also V$ and V_$?

These “$” views are called dynamic performance views. They are continuously updated while a database is open and in use, and their contents relate primarily to performance.

The actual dynamic performance views are identified by the prefix V_$. Public synonyms for these views have the prefix V$. You should access only the V$ objects, not the V_$ objects.

For almost every V$ view, Oracle has a corresponding GV$ (global V$) view. In addition to the V$ information, each GV$ view contains an extra column named INST_ID of datatype NUMBER. The INST_ID column displays the instance number from which the associated V$ view information was obtained.