what is the difference between DRD and Root Mirror Disk using LVM mirror ?

You cannot really compare...
LVM mirroring (MirrorDisk/UX) is using permanently 2 disks, so if any of the two fail you have your system still running...
DRD, is a cloning utility letting you be ableto clone your root disk and modify the clone... you can boot from...
The diference is if your root disk fails, your system will crash... but yes you reboot on the clone after, but in mirror/UX there is no non planified downtime and if you are using hot swap disks, there is no downtime at all...
Now if you are testing updates you see the difference: in mirror/UX you cannot come to a previous situation since both disks are affected unless you brake the mirror...

I believe you know about Mirror.
DRD is for normally OS patching like Solaris Live Upgrade..
We can do OS patch on DRD on business hours and then just reboot from DRD disk to get a patched OS.
It saves OS downtime.

Cheers,

DRD is a single point-in-time clone of your system.

LVM Mirror is a real-time mirror, equivalent to RAID1.