Save button of a formview within an UpdatePanel, causes page to refresh completely when clicked, how to avoid this?

A PostBack trigger will always refresh entire page as it calls all the PageLifeCycle events.

To add a trigger to a UpdatePanel, at the end of </ContentTemplate>
write the Triggers

<Triggers>
               <asp:AsyncPostBackTrigger ControlID="Button1" />
</Triggers>

If you want a PostBackTrigger then Replace the AsyncPostBackTrigger with PostBackTrigger in the above snippet.
Make sure that ChildAsTrigger property of UpdatePanel is set to True

Refer : https://msdn.microsoft.com/en-us/library/bb399001.aspx[^]