Replacepv simply moves all the logical partitions on one physical volume to another physical volume. The command is designed to make it easy to replace a disk in a mirrored configuration.

Migratepv is very similar. The biggest difference is that migratepv allows you to copy the LPs on a logical volume basis, not just on a physical volume basis. For instance, if you have a disk that has two logical volumes on it and you want to reorganize and put each logical volume on a different disk, migratepv can do it.

migratepv -l lv01 hdisk1 hdisk2
migratepv -l lv02 hdisk1 hdisk3

In this case, the logical partitions from logical volume lv01 are moved from hdisk1 to hdisk2. The logical partitions from logical volume lv02 are moved to hdisk3.