In English, I am cold and I feel cold mean the same. In Esperanto, can we use "malvarma" for a feeling of the subject, or we have to say something like Mi sentas malvarmon? (In order to avoid confusion with My internal temperature is low.)
It is not erroneous. Mi estas malvarma and Mi malvarmas are acceptable translations. (You will find that the sentence Ĉu vi estas varmaj? appears in Engholm's Infanoj en Torento.)
However, some people dislike it and consider it ambiguous: it might mean "I am a cold person" or "I am hypothermic" or something like that. (In particular, Ich bin kalt/heiss is inappropriate in German, and so native German-speakers are most likely to be bothered by it.) Therefore, alternative translations are:
Mi sentas min malvarma. I feel [myself to be] cold.
Mi sentas malvarme. I feel cold[ly].
Al mi estas malvarme. Al mi malvarmas. It is cold for me.