"urn:ietf:params:oauth:grant-type:jwt-bearer" is a URN defined as a JWT Bearer Token by OAuth 2.0 Authorization server (uses OAuth2.0 Authorization grant type ).
"urn:ietf:params:oauth:grant-type:token-exchange" is a URN defined as a JWT Bearer Token by OAuth 2.0 Authorization server (uses OAuth2.0 Token Exchange grant type).
Notes:
jwt-bearer means whoever bearing the JWT token shall be given access to the requested resource.
token-exchange is basically used for user impersonation and delegation purpose. Typically used by a support person who exchange his/ her JWT bearer token to impersonate as an end-user to request the end-user's resource to help in live-debugging of issue (or) exchange another JWT bearer token for a given token for a downstream systems.