Regarding the fact that both conducting electrons and holes can coexist in a semiconductor, I can understand how this might seem improbable. After all, since a hole is just the absence of an electron, why don’t the electrons simply fall into the holes, neutralizing both?
The answer here is that electrons do in fact fall into holes; this is called recombination. But this recombination can potentially take a long time because semiconductors have a band gap. A band gap is a range of energies that an electron cannot have in the semiconductor, and it exists due to the quantum mechanical (wave-like) nature of electrons in the crystal. A consequence of this is that if an electron is in a state above the band gap energy range (in the “conduction band”), and a hole is in a state below the band gap (in the “valence band”), then the electron needs to lose a lot of energy at once to “jump” the band gap and recombine with the hole. Since this process can take awhile, electrons and holes can coexist for some time (but not forever; this is a non-equilibrium state). In a solar cell, the energy from absorbed light promotes electrons from the valence band to the conduction band. This leaves electrons and holes both in the semiconductor at the same time. The trick for high efficiency is to collect those conducting particles before they recombine.
Now you asked about doped semiconductors, and in particular boron-doped silicon. The way to think about dopants is that they are little defects in the crystal that either steal electrons from the valence band (leaving behind holes) or they put extra electrons in the conduction band. Boron steals electrons from the valence band. But here’s the thing: if all of the electrons are in the valence band, none of them have any room to move (so to speak, it’s related to the Pauli exclusion principle). So a full valence band is inert, it doesn’t conduct, similarly to how in a crowded room no one can move around easily. As boron takes electrons out from the filled band, the other ones can move more easily. In a sense, during conduction, electrons are in fact continually falling into holes within the same band, but since the electrons that moved will leave new holes where the electrons had been, it is as if the holes are moving.