If a tv show is cancelled than will they still sell DVDs of all the seasons of the show that was cancelled?

Usually they will. The decision to cancel a series is usually made by the network, who thinks that the ratings for the show no longer justify keeping it on the air. The rights for the DVDs are owned by the studio which produced the show. So it's often worth it for them to put out some DVDs and try and make some money off of the show even though it's not airing. The exceptions to this is if a show is canceled early in its run. If only a couple of episodes were aired then they probably won't bother to put out a DVD of it.

It Depends on the sales/Copyrights

But usually they still sell the DVD as long as it wasn't canceled at the first season

Mostly those that have a full season( at lest one season not half or less) like,

Vegas

The Last Resort

Usually, unless it was so unpopular even that would be a money losing opportunity.

If the studio/producers think it will be profitable, yes. Otherwise no.