MXBeans are just a special kind of MBeans. The main difference is that MXBean restrict the data types, so that they are “more compatible” with potential clients.
As example: a MBean can expose attributes of a data type Foo. Now the client also needs to have this type Foo to make sense of the attribute.
The MXBean tries to restrict the data types to those “already available” – java.lang.* etc.
See this tutorial