Genocide is a very loosely used term nowadays. People use it to describe events in the past with great aplomb. Some historians refer to Julius Caesar's taking of Gaul as Genocide since he killed thousands upon thousands of Gauls to fulfill his goal.
Technically they are right that he committed genocide, but in another aspect they are wrong. Julius Caesar waged a 10 year campaign against Gaul to take the land, not to kill as many people as he could out of hatred for them.
Now, some of you might be confused on the relevance of this little side note, but rest assured that there is a valid reason for this
Alright, here are some arguments professing to Revan's innocence:
He was trying to save the galaxy when during the Mandalorian Wars.
The exile was the one who ordered the Mass Shadow Generator to be used.
And here is what I have to say to them:He saved the Republic and wasn't fighting to kill the Mandalorians for the sake of killing them.
To the first, it is true that Revan was trying to save the galaxy, but for what purpose? We all know thanks to Kotor that Revan defeated the Mandalorians, disappeared for a while and came back as a powerful Sith Lord. We can safely assume that he was already going down the path to the Dark Side before he left for the Wars.
Some examples of this are the shared dream with Bastila on Dantooine where you see Revan and Malak walking in the ruins on Dantooine with Malak confessing doubts about their path.
That means that we can also safely assume that Revan was going to war not only to save the galaxy, but to find a deeper reservoir or power. And it can also be said that he went to war simply for the sake of crushing the Mandalorians which were a threat at the time to his power should he find what he was seeking.
To the second argument, Revan ordered the Mass Shadow Generator to be built and he ordered the exile to use it. So, the exile didn't order the deaths of thousands, he was following Revan's orders to kill the Mandalorians on Malachor V.
To the third argument, Revan fought to protect his power, not to save the Republic. Once he found what he sought and tasted a small sip of the power of the Dark Side, he wasn't satisfied with the little glimpses of power that was offered by most paths.
To secure his power and crush those who didn't strongly believe in the path he was choosing, Revan ordered the deaths of thousands.
Remember HK-47's words in TSL? He states that those who died on Malachor were those who didn't strongly support Revan's cause or openly opposed him. The Jedi who died there weren't Revan's friends, but those who didn't approve of his methods but had joined the war effort anyway.
So, given all of that, Revan is guilty of genocide by modern and historical definition. He went to war for the sake of killing Mandalorians. He killed those who opposed him and raked up a death toll that most armies are hard pressed to match.