That the PM could be the MF i doubt. Because the MiG-21 came in 4 generation. The first generation was a daylight fighter without radar. The second generation was designed as interceptor with radar. The P was the symbol of that generation. It stands for perekhvatchik, interceptor. The third generation was multi role capable. Thatswhy the M for export models and S (saphire) for soviet used versions. The bis was the 4th generation.
I think the PM is a never existing version, like the PFMA, which you find often in the literature, but not in reality.
The PFL i found in Gordons book. I think he is wrong, because he cant tell us for what the L should stand. Topeczer repeated only what Gordon wrote. Why i say that? Because all letters have a meaning. P stands for radar equipment, F for stronger engines, S either for SPS System (border layer blowing system) or Saphire radar. T stands for more fuel (toplivo), -13 for equipment with K-13 missile. FL means "Frontline".
The single letter L has no meaning. It could have been used for "light" (lyokhki), but the plane Gordon mentioned was not lighter that the average MiG-21PF. It was a little bit heavier.
The V has a meaning, it stands for vetnamski (vietnamised) and was an indication for improved corrosion protection and additional protection against water and wet weather conditions (for instance the cooling slots in the radar cone were sealed). Gordon wrote, that the designation PF-V is wrong and was never used. And he is right. The designation was PFV, without "-". In russian laguage it is written so: МиГ-21ПФВ.