"Likely Voters" is a subset of "Registared Voters" and in polling standards is usually considered to be a higher standard.
Each polling agency has it's own methods of determining which registared voters they talk to are likely to vote.
Both LV and RV are a higher standard than "Adults".
I understand that LV's are a subset of RV's, however when you see a poll stating the sample is comprised of RV's it indicates the polling organization is using purchased lists of RV's to contact the saaple group. In other words their is no question the LVs are RVs but when I see LV's I'm not sure they are RV's and without a purchased list the pollster cannot be 100% certain.