1) Open SDK Manager (In Android Studio, go to Tools > Android > SDK Manager) and Download Intel x86 Emulator Accelerator (HAXM installer) if you havenot.
2) Now go to your SDK directory (C:\users\username\AppData\Local\Android\sdk, generally). In this directory Go to extras > intel > Hardware_Accelerated_Execution_Manager and run the file named “intelhaxm-android.exe”.
In case you get an error like “Intel virtualization technology (vt,vt-x) is not enabled”. Go to your BIOS settings and enable Hardware Virtualization. Use the installer from this link. If you are using Avast, disable “Enable hardware-assisted virtualization” under: Settings > Troubleshooting. Restart the PC and try to run the HAXM installation again
3) Restart Android Studio and then try to start the AVD again.