MVD is when there are microscopic vessels throughout the liver that are causing some of the blood to bypass the liver and not get filtered though, this is what causes the elevated BA, alot of the time MVD dogs are asymptomatic and live normal lives,some just need a diet change, and there are others that are very affected.
Portosytemic Shunt (LS) is when there is one larger vessel that causes all or some of the blood to bypass the liver. LS can also vary in severity but usually there are symptoms, and most cases it needs to be corrected for the dog to live a normal life
here is a good link that explains more
PSVA and MVD Research Summary
and another
http://www.yorkieangelpatrol.com/mvd.htm