Obviously the difference is an x, and its meaning some years ago was (according to Professor David L. Mills): Dennis Fergusson intended the “x” as “experimental”. I got maybe twenty messages over the years suggesting the x was not appropriate for code in use over a decade and I dropped it for NTPv4. See the paper on NTP history at http://www.eecis.udel.edu/~mills/papers.htm. In practice xntp refers to an implementation of version three or older while ntp refers to implementation of version four (or later).