I think that K refers to absorbed energy. The V or U is attached to indicate the type of impact test carried out (V or U notch).
The value would be reported as: KV 121J indicating absorbed enrgy of 121J using a standard V notch specimen, using a standard test condition ( test machine with a nominal machine energy of 300 +/- 10J.
Further details in EN 10045 part 1.
Regards
Andy