I don't think it would be technically correct to classify it as a genocide. To make it a genocide there would have to be an attempt to completely wipe out some defineable ethnic or tribal group. I don't believe that was what was going on.
Stalin's manufactured famines in the Ukraine could be fairly classified as genocides, however.
If not - what was that?
Mass murder. Not all mass murders are genocides. That doesn't make them any better, though. IMHO, all willful mass killings of the innocent are equally bad; the only objective manner of comparison ought to be in the number murdered.
I don't buy the argument that some forms of willful killing of the innocent are morally worse than others. There is no moral distrinction, in my book, between someone who kills 3 million because he's trying to wipe out some ethnic group or he because they refuse to accept his ideology. Both are equally bad.
Legal definition of genocide from the UN Convention on the Prevention and Punishment of the Crime of Genocide is:
any of the following acts committed with intent to destroy, in whole or in part, a national, ethnical, racial or religious group, as such: killing members of the group; causing serious bodily or mental harm to members of the group; deliberately inflicting on the group conditions of life, calculated to bring about its physical destruction in whole or in part; imposing measures intended to prevent births within the group; [and] forcibly transferring children of the group to another group
Since Soviets goal was to eliminate the Polish officers corps and the intelligentsia, I think it does qualify as a genocide.