Polycythemia is the presence of a larger than normal volume/amount of red blood cells in the blood stream, i.e. abnormally high PCV/HCT. Polycythemia can be primary (abnormality of the bone marrow/hereditary) or secondary (unnatural increase in erythropoeitin for various reasons). Transfusions can also cause polycythemia.
Certain breeds of dogs such as sighthounds (e.g. greyhounds) have naturally higher PCV/HCT levels, therefore higher than average levels are actually quite normal in these breeds. For this reason, they make very good blood donors.