The XM prefix indicates that it's a special-order item for a specific customer, usually military or law enforcement, loaded to their specifications. It almost certainly uses the same bullet as the regular HST (of the same weight), but the case might be plain brass instead of nickel-plated, it might be loaded a little bit hotter, it might use a different compound to seal the primer, it might use a different primer, it might use different propellants, or any combination of the above.
In most cases, the customer ordered more than they ended up wanting, and decided to dump the excess on the civilian market. It won't be exactly identical to regular HST, but it should be fine for you to use, as occasionally the manufacturer will tweak the specifications of the regular ammunition anyway.
