Fava beans (Vicia faba), also called broad beans or horse beans, are large beans that can be eaten raw from the pod. They're very good with pecorino (Italian sheep's cheese).
Both fava and bean are from the Proto-Indo-European root *bhabh-eh₂- "broad bean". Latin faba, Italian and Portuguese fava, French fève, Spanish haba.
The variant form *bha-un- became Proto-Germanic *ƀaunō: Old English bēan, Old Norse baun, German Bohne, Dutch boon. The Germanic words have been extended to cover any kind of beans, but the Italic words seem to still refer specifically to Vicia faba.
So both elements of fava bean are derived from the same root. Other phrases like this are chai tea and golden yellow.
source: The American Heritage Dictionary of Indo-European Roots, Calvert Watkins, 2001
The Oxford Dictionary of English Etymology, 1991
Multilingual Plant Name Database