I believe this is how the current situation came about:
Back in the day, completing a one year rotating internship was required prior to residency for both ACGME and AOA for both MD and DO students. That first year was called the "internship" and you were an "intern" not a "resident" yet. Then in the '70s MD residencies began dropping the requirement of a rotating intership in order to streamline post-grad education. Now they are not required for MD residencies but are available. So now days the term "intern" means first-year resident, so it is basically an obsolete term for someone doing an MD residency. However, most AOA residencies still require a rotating internship year. These are true internships - like what MD's did pre-'80s. So yeah, you do lose a year to your MD counterparts (if you go AOA). This is a big complaint among many DO students right now (ie, "Isn't the AOA trying to ATTRACT students to their residencies?").
There is another option for certain specialties (IM, EM,...) called "fast track" internships which are internship specializations that are sometimes applied to your residency years. I don't know a lot about them except they are competitive. This seems to me to be the same thing that happened with MD residencies 30 years ago. I think that over the years we will see certain specialties fast tracked across the board.
As stated above, if you want to skip the whole internship thing, do an MD residency. Unless of course you wish to practice in one of the five states above that have it written in state AOA laws that to be board certified you must complete an AOA internship. You can then do an MD or AOA residency. Sucks to live in those states!