Gardner, who is the younger brother of Sunderland midfielder Craig, is highly regarded at Villa and is tipped for a big future at Villa Park.
Sky Blues boss Andy Thorn was delighted to land Gardner after reportedly beating off a host of clubs to the midfielder's signature.
"I am delighted to get him," Thorn told the Coventry Telegraph.
"Although I have been looking for a bit of experience, this was an opportunity I couldn't pass up because Gary is such a talented lad.
"He's over 6ft, has got a great engine, can tackle and he scores goals from midfield.
"He's an England Under 21 international and a proper talent and we've beaten off a host of other clubs to get him because he likes to play football and likes the way we play, so he should be a good addition for us."