http://en.wikipedia.org/wiki/List_of_genocides_by_death_toll Mao Zedong is believed to be responsible for 34,300,000–63,784,000 deaths and Joseph Stalin for 23,000,000–60,000,000.
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.
The intent is important, no matter what you might believe. That's why the killings in Srebrenica were recognized as genocide, even though "only" 8000 civilians were killed. It does not matter if 10 million or 10 thousand are killed, the intent is all that matters.
What follows is that the starvation caused by Maos policies were not a genocide, no matter how many people died, simply because the government didn't intend to kill all Chinese systematically. Quite to the contrary, they seriously believed that collectivization would end the starvation that has plagued China for decades.
The same applies to most deaths caused by Stalins regime. In some cases there was genocide going on, but the largest part of the victims of his regime were due to failed economic policies (starvation) and political persecution.
Whereas Hitlers policy was to exterminate ethnical or religious groups he deemed worthless, like the Jews, the Slavs, the Roma and many more. He clearly had the intention to exterminate those groups, he even stated it publicly.
And where is this written as Canon?
Politics is also performed with imperfect predictions of the future. If policies pursued in good faith result in the death of millions, the people implementing them are not as responsible as those for whom death is the policy.
Now, if their intent was to starve the masses to further their goals, you would have a point.
> And where is this written as Canon?
In the minds of the survivors.
> If policies pursued in good faith result in the death of millions, the people implementing them are not as responsible as those for whom death is the policy.
History doesn't see it that way. People don't see it that way. Hitler and Stalin are equally reviled, even though Hitler's genocide was intentional and Stalin's was a side effect of "good intentions" -- a Communist revolution.
> Now, if their intent was to starve the masses to further their goals, you would have a point.
Like Mao and Pol Pot? Both were able to explain their programs as "for the good of the people", and both pursued their programs long after their effect was perfectly clear.
For a discussion on the historiography see http://www.nybooks.com/articles/archives/2011/mar/10/hitler-...