XT vs AVA Plugins

The Harrison XT plugins are developed entirely for use inside Mixbus:   they are pre-installed in Mixbus (although unlicensed).  Updates to the plugins are rolled into Mixbus so you always get the matching (tested together) versions.   The XT plugins generally support a wider range of computers; the same range of OS'es that Mixbus itself supports.

The AVA plugins are more traditional plugins that operate in "any" workstation.   Some of them share exactly the same sound/DSP as their XT counterparts.  But the AVA plugins are a slightly newer generation.  This means (a) they have some update GUI features, such as a realtime analyzers,  but  (b) they are generally optimized for newer, faster computers.   The AVA's typically cost more than the XTs.