CVM

CVM is an archive format used in the PS2 versions of Persona 3 and Persona 4. It was developed by CRI Middleware and based on ISO9660. It is also known internally as "ROFS" format. Essentially, they are ISO files with a special header. Due to this, most programs that can normally extract ISOs can also extract CVMs. All game data (except the executable) is stored in CVM archives.

cvm_tool
Commandline tool ROFS tool v0.02 was released by roxfan in 2010. It can "split" the CVM into separate header and ISO files, so that you can edit the contents of the archive and then "make" a new CVM.

Use the following commands to: Extract ISO file from a ROFS volume.  Make a ROFS volume from an ISO file and header file. 

The tool can be found here.

For more information on editing ISO files, see ISO.

Mod Compendium
TGE's Mod Compendium can automatically merge multiple mods into new CVM files and generate a patched SLUS based on them. You can download it here and read this guide on how to use it.

Be advised that the Mod Compendium does not create ISO files from a CVM like cvm_tool.