Aliens' chitinous exoskeletons are capable of resisting low-caliber ballistic rounds, although not much more than that. Most weapons are capable of easily penetrating their defensive armour, as there is only so much punishment it can take. In Aliens, handgun rounds simply ricocheted from the head of one of the warriors, although they have a tougher exoskeleton then most other xenomorphs.
Today's standard issue military assault rifles, such as M4A1's would more than likely be able to shred through a xenomorph's exoskeleton with relative ease. The pulse rifles in Aliens fired light armour piercing rounds, & were easily able to cut down their defenses, so I imagine many modern day weapons would be able to as well.