In short: Usually the suffix “הם” HM is used to indicate “them” or
“their”, as in “אליהם” )LYHM or “כליהם” KLYHM. Sometimes, though, the
suffix used is “מו” MW, e.g., Deut 33:2 “וזרח משעיר למו”.
Is there any known grammatical difference between the two forms, or is
the -MW form usually taken to be simply a poetic style?
—Joel
[b-hebrew] -HM vs -MW suffix,
Joel C. Salomon, 02/14/2012