Every PE/VC firms has a stake in the business, if he sees the business is doing fine and will be able to sell his stake at a good profitable price then he can exit from the business in any of the following methods
IPO: When the company makes a initial public offering, the PE/VC firm will sell his shares to the allotted public at the price decided by the company.
Secondary Sale
Buyback: The founders buy back the stake from the PE/VC firms.
Public market Sales: The PE/VC sells his stake to anybody in the public, it could be banks, individuals, companies etc.
