What is the Difference Between Mechanical And CV Carb?

The difference is in how the venturi size is changed.

On CV (constant velocity) carbs - when the throttle is twisted, the cable opens up the butterfly valve, which allows air to be drawn into the mouth of the carb. You control the butterfly which allows air in, which controls the slides, which controls the  air/fuel  mixture entering the engine.

Mechanical carbs have the slide directly attached to the throttle cable so your throttle controls how high the slide goes.

Mechanical slide carbs are far more common in  high-performance  situations where the throttle is WO most of the time. 
CV  or Constant Velocity carbs offer the best real-world performance for day to day riding where your throttle isn't pinned all the time.

the CV carb allows much more precise metering and thus, a less boggy engine under load conditions. A CV carb lets you twist the throttle wide open and you don't lose power doing this. With your typical mechanical or round slide carb, for example, in a hill climb situation, you twist the throttle wide and what happens is the engine loses vacuum (because it's under a heavy load) and you will lose performance. "Constant Vacuum" carbs have a vacuum tower (the slide and diaphragm assembly) that enables them to detect engine load and make changes to the mixture adjustment accordingly, so you end up being able to accelerate through loads much better. With a faster response. 

The Honda Shadows all have CV carbs unless they are EFI.