How the risks will be defined & managed in agile life cycle ?

In the same way you manage the risks in other approaches. Agile is not a life cycle. Agile is an approach. You can use Agile with waterfall, iterative, incremental, iterative-incremental, spiral, V, etc life cycle. No matter that I know (and I debate a lot on the matter where I was part of the group or authors and reviewers of the PMI´s standards) the PMI has called the iterative-incremental like Agile life cycle which is one of the biggest mistakes (not because I am saying that, you can find a lot of research on the matter). I wrote all or that because you need to think your method for doing things driven by architecture. Then you have components. Risk and Issues management is a component that you can insert a a function inside your project management process. With that said then you can choose your approach, your life cycle, and your method based on your life cycle. For example, Scrum is a framework, not a method. Then you have to fill it up with the tools and techniques that best fits for your current situation and the desiere situation. For example, for requirements, you can choose to use user stories or use use cases or use Yourdon structured analysis techniques and tools. The same for other type of things. Or if you go to other well known methods like DSDM you will find risk and issue management explicit defined inside it. So, again, do not fail in the trap. Define your way or working.

I think there's a question behind Amjad's question - who is responsible for risk management when using Agile approaches? The answer will vary, based on how your company is organized - in some approaches, you have a project manager, in others, the PM role is divided between two, or more, roles.

Using Scrum training as an example, Risk Management was not covered in either the CSM, CSPO, or A-CSM classes I've taken (from different providers). The following is a simplified summary of how risk management could be applied.

In practice, the PO should be aware of organizational risks, have a role in identifying and managing them, and make the team aware of them. This can start before the Scrum team exists, when working with the business to identify product requirements. The CSM should be able to identify schedule risks, share them with the pigs and chickens, and work with the appropriate people to address them. The team should call out risks to their individual tasks.

This response is not intended to be comprehensive; there is more to risk management than I've addressed. My point is that in some Agile approaches, Risk Management is distributed amongst multiple roles instead of sitting with the project manager.