EV is the prefix used by the company formerly known as DEC to describe one of their Alpha CPU cores. The breakdown for the EV cores is as follows:
EV4  : 21064
EV45 : 21066a
EV5  : 21164
EV56 : 21164a
EV6  : 21264
EV7  : 21364

EV, a reference to the famous DEC Tech Report at: http://www.research.digital.com/wrl/techreports/html/TN-13/ , stands for Electro-Vlasic.