The Institute for Government documents the possibilities in these circumstances.
It is possible for a deal to be just approved at the EU level following a decisions by members states governments.
Member state governments could collectively decide to limit ratification at the EU level, so in the Council of the EU and European Parliament. The 2019 Japan–EU trade agreement was ratified as an ‘EU-only agreement’ though it included provisions of shared competence. The UK’s negotiating mandate suggests the government wants an agreement similar to this.
This is considered unlikely. Moving forward under that understanding we come to provisional ratification.
The EU could provisionally apply the UK–EU treaty pending full ratification – but the Council of the EU would need to consent to provisional application before 11:00pm on 31 December 2020.
The European Parliament has asked the Council of the EU not to agree to provisional application until MEPs have had the chance to scrutinise and vote on a trade deal. However, this is not a legal requirement.
Under provisional ratification the treaty would come into force as if it were fully ratified, with that state expected to follow later. To date no treaty that was provisionally ratified has failed to obtain full ratification later.
The full page discusses the time table of previous ratifications, how certain provisions were delayed from taking effect and so on. The full page was last updated on Nov 23 2020 at time of writing, so is up to date by any reasonable understanding.