Students are often confused by the MPx and xPM Extreme Project management approaches. The following two links may shed some light on this. The first is a link to an article by Wysocki on the Extreme and Emertxe Project Management Models. These are two models used when solution paths are unclear. The second is a good blog post which summarizes the 5 life cycle approaches. It is based on experiences in the construction industry. As such the Extreme models are not used but the article does discuss them including the xPM and MPx approaches.
xPM and MPx Project Management Models
Blog Summary of 5 Project Management Life Cycles