Multiverse separate inventory per worlds
Make sure you install the plugin multiverse-inventories! Multiverse-inventories is a extension for multiverse to configure seperate inventories. If you installed it, you have to configure it.
How you are able to configure it you find in the config.yml as a comment. After you finished the configuration you have to reload the file with the command /mvinv reload.
If you have any problems while installing or configurating take a look at the offical wiki of mulitverse-inventories.