English, from a reduction of the Hebrew name, Kerenhappuck, the youngest of Job's three beautiful daughters [OT Book of Job 42:14].
Both Keren and Kerenhappuck were fairly popular names in Scotland in the 17-18th centuries although the spelling of the latter was extremely variable [see Kerenhappuck].
Keren is quoted by ONC as being a "more manageable" diminutive of Kerenhappuck which is certainly true. There is no connection with the modern Karen which is from the Danish variant of Katherine.
A Latinate version of Keren is Kerena, found in old documents, with appropriate case endings.