Fervency, n.
Etymology: [Cf. OF. fervence. See Fervent.]
The state of being fervent or warm; ardor; warmth of feeling or devotion; eagerness. When you pray, let it be with attention, with fervency, and with perseverance. Wake.
Shades of meaning
Fernald has no entry of its own for fervency; he lists it under enthusiasm.
He lists it without comment; the entries above explain its neighbors.