Hi Shruti,
Below class is to send email by using the apex. Pass the EmailAddress, Subject and Body to the below class, then it will send the email.
As Previous and new owner as storing in seperate fields, with that field query the User Object and get the EmailAddress and pass that emailAddress to below class.

public class EmailManager {

   // Public method
   public void sendMail(String address, String subject, String body) {
      // Create an email message object
      Messaging.SingleEmailMessage mail = new Messaging.SingleEmailMessage();
      String[] toAddresses = new String[] {address};
      mail.setToAddresses(toAddresses);
      mail.setSubject(subject);
      mail.setPlainTextBody(body);
      // Pass this email message to the built-in sendEmail method 
      // of the Messaging class
      Messaging.SendEmailResult[] results = Messaging.sendEmail(
                                new Messaging.SingleEmailMessage[] { mail });
      
      // Call a helper method to inspect the returned results
      inspectResults(results);
   }
   // Helper method
   private static Boolean inspectResults(Messaging.SendEmailResult[] results) {
      Boolean sendResult = true;
      // sendEmail returns an array of result objects.
      // Iterate through the list to inspect results. 
      // In this class, the methods send only one email, 
      // so we should have only one result.
      for (Messaging.SendEmailResult res : results) {
         if (res.isSuccess()) {
            System.debug('Email sent successfully');
         }
         else {
            sendResult = false;
            System.debug('The following errors occurred: ' + res.getErrors());                 
         }
      }
      return sendResult;
   }
}

Thanks,
Narendar