  We study the parametric decay instability of parallel propagating Alfv\'en
wave in a low-beta plasma using one-dimensional fully kinetic simulations. We
focus for the first time on the conversion of the energy stored in the initial
Alfv\'en wave into particle internal energy, and on its partition between
particle species. We show that compressible fluctuations generated by the decay
of the pump wave into a secondary ion-acoustic mode and a reflected Alfv\'en
wave contribute to the gain of internal energy via two distinct mechanisms.
First, the ion-acoustic mode leads nonlinearly to proton trapping and proton
phase space mixing, in agreement with previous work based on hybrid
simulations. Second, during the nonlinear stage, a compressible front of the
fast type develops at the steepened edge of the backward Alfv\'en wave leading
to a field-aligned proton beam propagating backwards at the Alfv\'en speed. We
find that parametric decay heats preferentially protons, which gain about 50%
of the pump wave energy in the form of internal energy. However, we find that
electrons are also energized and that they contribute to the total energy
balance by gaining 10% of the pump wave energy. By investigating energy
partition and particle heating during parametric decay, our results contribute
to determine the role of compressible and kinetic effects in wave-driven models
of the solar wind.
