RE: JR DS821 HV & DS 821's
HV just stands for "High Voltage" and in the case of servos it refers to a servo that can take unregulated Li-Ion/Li-Po voltages. The nominal voltage on a two cell pack for those chemistries is 7.4 volts but they come off the charger at 8.4 volts. Pretty common as an option in the higher end digital servos but this is the first I've seen it on a servo that didn't cost over $100.00. Just be aware of the recommended use limitations on the 821 servos like model size/weight/engine restrictions.