SV - Scalar Value
IV - Integer Value
NV - Numerical Value
RV - Reference Value
PV - Pointer Value (like *char, for example)
LV - I haven't encountered this one, maybe it lives in illguts
GV - glob value. The glob is (I think) shorthand for global
AV - Array Value
HV - Hash Value
CV - Code Value (think subroutine reference)
FM - Format
IO - Filehandle