Please someone can tell me what is the difference between redundancy PLC & Hot Redundancy PLC.

I suggest you reading the following topic:

Cold, warm and hot redundancy: determining how much you need

 to add extra points regarding SIMATIC products related to this topic:

1. hot redundancy is a model you can see in S7-400H, it is used in critical processes such as power plants,gas production plants and high speed rotary machinery.In case of master CPU failure, the standby CPU can take control in 10ms order with previously updates from process.In this manner, the change over is bumpless.
   
   
2. For warm redundancy , you can check software redundancy.It is not as fast H applications but the switch over delay can be tolerated for the most sluggish process models.
   
   

I hope it helps,

hdhosseini