A side event is part of the official conference programme and is selected by the UNFCCC secretariat. A pavilion event is put on by whoever leased that pavilion, on their own terms. The UNFCCC is explicit that pavilion events are not part of the official programme.
The practical difference is who decides. For a side event you apply to the secretariat through its online system, and selection weighs relevance to the negotiations, along with regional, thematic and gender balance. For a pavilion event you persuade the pavilion host, who answers to nobody but themselves and their funders.
That makes pavilions far easier to get into and far more variable in quality. It also makes them where most of the interesting conversation happens, because a host can programme a subject the formal agenda has no room for.
Neither route gets you into the building. Accreditation is separate, and a pavilion host generally cannot provide it. Confirm your badge before you confirm your speaking slot, not the other way round.