ek diagram heavy
Heavy and light holes (also - holes in so-called split-off band) are just different types of holes (like different types of atoms or molecules occupying the same volume). Concentration of heavy holes is much higher than that of light holes, due to their larger mass and thus density of states.
The energy-wavevector (E-k) relationship is shows the dependence of total energy (i.e. kinetic plus potential energy) on the wavevector. Wavevector k is defined as particle (electron, hole,...) momentum divided by Planck's constant (hbar). Since the absolute value of the potential energy is unimportant, you can change the scale so that E=0 at k=0.