Hallow states "Celestials, elementals, fey, fiends, and undead can't enter the area..." but it doesn't say what would happen if someone tried to summon some into the area. Would the spell fail? Would the elementals appear but then be forced to leave?
Would the ruling be the same for both Conjure Minor Elementals and Conjure Elemental? The latter uses a source of the element (e.g. a fire, what if the fire is in the Hallow area?) while the former just makes some minor elementals appear from an unspecified source.