If you're looking to watch the entire series of Beverly Hills 90210, you may be disappointed to find that Hulu does not have all the episodes. The only way to watch the full series is to purchase the DVDs. Dailymotion has some of the episodes, but not all. Additionally, due to music licensing issues, some scenes are missing from the DVDs.
The group R&B Color Me Badd had a significant presence in 90210, but it appears that negotiating music rights got in the way of broadcasting episodes. It's even possible that some scenes within episodes were cut due to music licensing issues. For example, when watching Season 4, there was a scene that was cut out that had music in it. Finally, Season 10, which is the last season of the series, has episodes 2, 6, 10, 12, 14, 16, 18, 20, 24 and 27 missing.