Before you can write directly to a partition, you must first ensure that it is unmounted.
Linux will not notice if you write directly to the disk behind its back (and, more importantly, will assume that this will not happen---it will likely get very confused if you try modifying a mounted file system.)
So, the correct procedure would be as follows:
umount /dev/xvdk
gunzip –c mySda1.gz | dd of=/dev/xvdk
mount /dev/xvdk
and again for /dev/xvdl
.