I still haven't decided when the purge actually happened...my initial thought was that the purge was before Kelvin got there, since it sure seems like the Swan was the one place the Hostiles could not take over. I figured that Dharma was still manning and supplying the Swan after the purge due to the important nature of keeping it going. Now I'm not so sure when to date the purge. There's definitely some inconsistencies with what we've been told about the purge...like Mikhail claiming Dharma initiated a war with the Hostiles (which he called the purge) while it seems clear from Ben's flashback that the Hostiles are the ones who "purged" Dharma. Read through this for the details of what we've seen so far...

The Purge - Lostpedia