Location: In the bothy, 7 chains down the line from Dodman's Lane level crossing, in the nation formerly known as Great Britain - possibly to become "South Scotland" or "Non-Caledonia" from September 2016. Kettle's on.
This is really a question for the "Electrical Engineering" Section of CR4.
Do these codes appear on the same or different drawings or installations?
Is there any "key" drawing or legend which defines the codes?
Where are PT and RVT connected on the "single line" or other drawing you have?
Depending on the customary description, originally British or USA practice, VT = Voltage transformer and PT = potential transformer both mean an instrument transformer which accurately transforms the voltage between some places in a high voltage installation to 110 [British] or 120V [USA] for a given High Voltage; e.g. for 11 kV to 110 V the instrument transformer would have 100:1 ratio or 14400/120V with 120:1 ratio
RVT has no standard meaning - it can even mean "Rotary Voltage Transformer" which actually meant a DC commutator rotating motor-generator which took 28 VDC from an aircraft battery and gave 250 VDC out.
In a place where PT means an instrument voltage/potential transformer, RVT may mean a transformer of moderate size with a secondary low voltage resistor load which is has the purpose of giving a resistive load at the high voltage side for earthing/grounding reasons. If it were a grounding transformer, it would usually have an earth symbol connected to one end of the HV winding.
If you want better than this guess, you must give more information!