There is only one file type that uses the .TXZ extension:
XZ Compressed Tar Archive
Archive created with Tar and XZ utilities; contains one or more files first archived using Tar and then compressed using XZ compression; compressed using a high compression ratio.
Beginning with Slackware 13, TXZ files are used to distribute Slackware Linux packages. They can be opened using various Slackware package management tools, including pkgtool and installpkg.
Due to the high compression ratio, TXZ files are typically smaller in size than .TGZ files.
Updated: September 5, 2009