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.