Hi Suraj,
You need to log a ticket to Salesforce to enable Person Accounts in Dev Edition.
To log a ticket -> Login into your Dev Edition -> Click on “Help” at the Top -> Click on “My Cases” -> Click On “Log A Case” -> Select the Case Reason as “Feature Activation Request” -> Select “Feature Activation” in “General Activation Area” and ask for the Person Account Enablement.
Hope this helps.
Thanks.