The concert will be the first time Grohl and Novoselic have played live together in almost 20 years, following the death of Cobain in 1994.
Novoselic appeared on the last Foo Fighters album.
McCartney is elated about the historic event.
"[I phoned Dave and he invited me to come over and] jam with some mates," McCartney told British newspaper The Sun.
"I didn't really know who they were. They are saying how good it is to be back together. I said, 'Whoa? You guys haven't played together for all that time?
"And somebody whispered to me 'That's Nirvana. You're Kurt.' I couldn't believe it."
McCartney, Grohl and Novoselic will be sharing the stage with Jon Bon Jovi, Bruce Springsteen, Billy Joel, Alicia Keys, Kanye West and The Rolling Stones.
The show is expected to make history as the most widely distributed live musical event ever, with two billion people around the globe expected to tune in to the fundraiser.
- Cover Media