If the user makes any changes to xamgrid

i.e changes value in textboxes/combo box, once he resets the button , xamgrid should display original contents that were in grid before any changes were made

If you have already modified the data objects that the silverlight client has in it's datasource, you are going to probably need to go back to your server and re - get all the data that is in your data source.

The grid does not remember what you did.  You could possibly program that into your data objects to reset them to the last state they knew about prior to being edited, but that would be on the data object level.

I have each row ties to a checkbox

If the user selects few rows by selecting checkbox or deselects the grid should refresh only thos rows

The entries are not yet made to the database

Changes are just in the UI

Is it possible to just refresh those rows that were changed?

You would need to either roll back the changes via your data object, if you coded your data object to support roling it's data back or look to your back end data store and refresh your data values.  The grid changes the properties on the data object, it does not track it's changes to your data object.