How can I see the number of allocated pages in lvm logical volume?

Qubes OS private volume uses lvm copy-on-write snapshot. I want to profile VM behavior and see whether there are more write actions than I have expected.

I have tried lvs lvdisplay and none of them tells how many modified lv blocks exists in a copy-on-write snapshot.

I want to profile my VM and check whether certain operations have more harddisk write than I have expected. However I can only see the total used pages from the perspective of one logical volume, rather than the difference between the current logical volume and the base volume.