Tell me differences between polycythemia, polycythemia rubra vera and polycythemia vera?

Polycythemia: Polycythemia rubra vera and polycythemia vera are the same thing - a myeloproliferative disorder which causes production of too many red cells (and usually white cells and platelets also). Polycythemia or erythrocytosis just means someone has too many red cells, whether due to prv, high altitude, sleep apnea, emphysema, certain cancers, or certain congenital abnormalities of red cell production.