Original: Volume management treats PVs as sequences of chunks called physical extents (PEs). Some volume managers (such as that in HP-UX and Linux) have PEs of a uniform size; others (such as that in Veritas) has variably-sized PEs that can be split and merged at will.
Simplified: PVs are split into small chunks of even size (a default of 4 MB on HP-UX) called physical extents or PEs.