would like to connect my ec2 with DCV viewer

If this is Linux, you need to install a desktop environment first. For both Windows and Linux, install NICE DCV server software. You can connect using web browser or native NICE DCV viewer client.

NICE DCV login do not use key pair. You need to set a password for the login user.

Refer to https://docs.aws.amazon.com/dcv/latest/adminguide/setting-up-installing.html for installation instructions.

Alternatively, you can setup a new EC2 instance with NICE DCV server using CloudFormation templates at https://github.com/aws-samples/amazon-ec2-nice-dcv-samples or from https://aws.amazon.com/blogs/desktop-and-application-streaming/announcing-updates-to-nice-dcv-aws-cloudformation-templates/