Dear Abhinav,

There are two possibilities:

1) HAXM installer is not installed: To install please open SDK manager and install it. Please refer to the below screen-shot:

Android SDK Manager

After you download it and make sure you run the setup located in: {SDK_FOLDER}\extras\intel\Hardware_Accelerated_Execution_Manager\intelhaxm-android.exe

2) The other reason could be virtualization is not enable on your system: 

Please find the document attached that would guide you in overcoming the above problem.

Please feel free to revert if you face any issues while doing that. I will wait for your response.
177560