It says ἡ Χριστοῦ γέννησις and it means "The Birth of Christ"
The HXV is really Ἡ ΧΥ, with a line over the XY, the standard nomen sacrum (sacred name) abbreviation for Χριστοῦ.
Γέννησις means birth, and it reflect the wording of the Byzantine text of Matt 1:18. The critical text, however, there has γένεσις, which was the word you were thinking of.
Stephen C. Carlson, Ph.D. (Duke)
Post-Doctoral Fellow, Faculty of Theology, Uppsala