There are two types of heart valve replacement. Tissue type, and mechanical. First we'll see some pics of the mechanical type.
The picture above, are examples of the many different types of mechanical heart valves that are available. The draw back to mechanical valves is they can cause more blood clotting, which leads to higher chances of a stroke. But the chances can be lowered with medication.
The picture below is what a natural tissue heart valve would look like. The most commonly used animal tissues are: porcine, which is valve tissue from a pig, and bovine pericardial tissue, which is from a cow. Animal tissue valves are often referred to as heterograft or xenograft valves.
Reference: http://cape.uwaterloo.ca/che100projects/heart/files/testing.htm
