For Michael, there was no distinction.

See page 145 in the paperback edition of the novel, for a scene that takes place after the decision is made that Michael will kill Sollozzo and McCluskey....

Tom says to Michael:

You shouldn't let that broken jaw influence you. McCluskey is a stupid man and it was business, not personal.

Michael's response is too long to quote in it's entirety, but it's one of the best passages in the book, I think. Michael starts off by saying:

Tom, don't let anyone kid you. It's all personal, every bit of business. Every piece of shit that every man has to eat every day of his life is personal. They call it business. OK. But it's personal as hell.....