You can choose to refresh the data for a Power View sheet or view. You have no choice about refreshing the data model; if it changes, you get a message that Power View needs to refresh the sheet or view.
When you refresh the data, you can choose to refresh it for one Power View sheet in a workbook (Refresh) or for the entire workbook (Refresh All).
Refreshing the data goes all the way back through the data model on which the Power View sheet is based, back to the data source to get the latest data. So refreshing the data in Power View also refreshes the data in the data model, whether it’s the internal Data Model in the Excel workbook or an external data model the Power View sheet is connected to.