Ilya Somin reviews a recent book that takes up the question:
Many Ukrainians and some Western scholars argue that this was a case of genocide because Soviet dictator Joseph Stalin specifically targeted Ukrainian peasants for extermination. By contrast, the Russian government claims that Stalin was an equal opportunity mass murderer. The distinction matters because international law defines mass murder as genocide only if it was the result of an “intent to destroy, in whole or in part, a national, ethnic, racial or religious group, as such.” It also matters because of the ongoing debate over whether communist mass murders deserve as much opprobrium as those of the Nazis.
Anne Applebaum has the most judicious take on this question I've read in a must-read in the NYRB. Her core point: the very definition of genocide was created by Stalin to exempt himself:
Soviet diplomats had demanded the exclusion of any reference to social, economic, and political groups. Had they left these categories in, prosecution of the USSR for the murder of aristocrats (a social group), kulaks (an economic group), or Trotskyites (a political group) would have been possible.