When Nirvana reunites tonight in NYC -- with Sir Paul McCartney replacing Kurt Cobain on vocals -- Courtney Love won't be cheering ... she'll be yelling obscenities, because she's pissed about the gig.

Dave Grohl and Krist Novoselic are playing at the 12-12-12 benefit concert at Madison Square Garden -- their first show together in 20 years -- but Cobain's widow tells TMZ she is "not amused" about Paul's involvement.

Courtney is clearly more of a Lennon fan, than McCartney because she added ... "Look, if John were alive it would be cool."

Love is also upset at Krist and Dave for calling tonight's show a Nirvana reunion because she says Kurt was the heart and soul of the legendary band.

For the record ... Sir Paul and the guys will be playing a brand new song they wrote together. It's unclear if they will cover classic Nirvana songs.