A subgenre of Heavy Metal, pretty much single-handedly defined by the German speed/power metal band Helloween.
Happy Metal is characterised by an upbeat tempo, catchy vocal lines, happy-go-lucky lyrics, humourous musical ideas, and, more often than not, funny cover artwork; it's not as gloomy, ponderous and theatrical as much other 'true metal', though of course the grave growlers will label anything they want to dis 'Happy Metal'.
Other than Helloween, the only well-known happy metal act I can think of is Gamma Ray, who incidentally started out as a Helloween spin-off. Stratovarius are supposed to be Happy Metal, too, but I don't know any of their works. Some smaller bands have gone so far as to defining themselves as Happy Metal, such as Afterburn, Avalanche or Drop Out. (For some reason, many references on the Web point to Bon Jovi -- well, at their heyday, they played hard and fast and their music certainly was happy, but they don't really compare to Helloween. If there's a 1980s band to be called Happy Metal, it's Van Halen, without any shade of doubt.)
There is some real fun stuff to be found in the Gamma Ray and Helloween catalogues, such as Helloween's Latin hymn to the saviour, "Lavdate Dominvm", done high-speed, Norwegian-fjord-style or "The Game Is On", their anthem to Tetris, finally putting the shine of some eternal epic metal glory on everybody's favourite block-dropping game and featuring samples from the Game Boy version's original soundtrack. Don't forget Gamma Ray's classic rendition of The Pet Shop Boys' "It's A Sin" or their silly pop-style synthesizer antics in the intro to "Send Me A Sign", either.