When you mean inordinate desire for revenge....does this constitute in wishing bodily harm or simply wanting to scourge (maybe not literally) everything in your path?
FYI, parvity of matter means "smallness" of matter; i.e., things not so serious. If I am ticked with a man, and want to beat him up and down, or even kill him, then I have likely crossed over into the serious end of the spectrum. However, it also depends on what he did to elicit such a reaction - some actions naturally elicit a stronger response, and should (see the Temple and golden calf bit above).
If I am just generally ticked, for a lesser reason, but wish no one actual harm, it is likely venial (even if I have a hard time shaking it over minutes or hours). It is best, I find, to ask the priest about it when you confess it. Tell him about your sin of anger, and your specific desires. This will help him to know how best to counsel you. If it was mortal, you have to confess it; if it was venial, confessing it is good for you, too. You cannot lose either way.