Watford were reduced to 10-men as they were swept aside by Hull, ending their 12-game unbeaten run.
Michael Turner set the home side on the road to a sixth straight win with a powerful header after just 47 seconds.
Frazier Campbell doubled Hull's lead as he directed Henrik Pedersen's volley into the net, before Caleb Folan raced clear and slotted home for the third.
Moments later, Steve Kabba was shown a straight red card for a reckless challenge on Sam Ricketts.
Hull boss Phil Brown: "We've given ourselves a good chance. We are now three points ahead of them so that shows the progress we have made."
"The confidence is there for everybody to see, and the fans can see what we are trying to achieve.
"We need to rest some tired legs over the next two weeks. The performance was a big bonus."
Watford manager Aidy Boothroyd: "It was the third anniversary of me taking over this week, and that was without doubt the worst performance during my time here.
"We were beaten by a better team. They were better than us at the back, middle and up front. I've no complaints.
"We're usually good starters, but gave away two really poor goals, early."
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