AC voltages have been classified in various manners.  In earlier times, there were just two categories LV and HV.  As the level of voltages increases, there was a need for more levels.  However, there was ambiguity as to where each band ended and the other began.  For instance, 11kV can be MV in some systems and HV in another. 

The International Electrotechnical Commission has classified the voltages into the following levels(IEC 60038).  This classification system is fast gaining acceptance. 

Low Voltage           - upto 1000V

Medium Voltage     - 1000V to 35kV

High Voltage           - 35kV to 230 kV

Extra High Voltage  - above 230 kV.

In some situations, the term Ultra High Voltage is used to denote voltages above 800 kV.

In addition, the IEC defines a voltage band known as the Extra Low Voltage with a AC voltage less than 70 V.  See article here.