Cravenly is the adverb: you do something in a way that shows you completely lack courage. The meaning matches craven; this form just describes the action. Someone cravenly stays silent, cravenly blames a weaker person, or cravenly hides when they should step forward. Like the adjective, it is literary and a little old. In everyday talk you are more likely to hear they were too scared, or they backed down. Writers still use cravenly when they want the reader to feel the contempt, not only the fear.