Finally I've solved it. But let me add that I have faced the same problem even when I wanted to move a 160GB pv to 250GB pv, where partition setup was identical because sda partition setup was copied to sdb.
And when the same problem emerged with two 250GB hdds as well, I have opened this thread to ask for advice.
It seems that I simply didn't have free space on the new pv to store old pv...
So the idea was to decrease the size of original sda pv to 99% of max available space, so that sdb pv will have enough space for it, and it did the job, pvmove is now possible.