The 28-year-old has moved to the newly-promoted club on a three-year deal for an undisclosed fee following two years at St Andrews.
I m delighted to be here, he told the club s official website.
I feel that I m joining Hull City at an exciting time and the chance to play in the Barclays Premier League was also a huge attraction. It s something that I ve been lucky enough to do before and I wanted to get back to that level.
Hopefully I can help this team to have a good season and push as high up the table as possible. I can t wait for Chelsea on the first day of the season now.
Davies most recently featured in the top flight for Aston Villa in 2010 but has since been playing his football in the Championship.
He started 40 games for Birmingham last season, scoring six times.
His move to the KC Stadium signals the second signing of the close-season for Hull, who have also brought in Maynor Figueroa in a bid to strengthen their defence.