Hive which is a UK based company and part of British Gas, has been making its own smart home devices for a few years now and thanks to marketing from British Gas, it is popular here in the UK, it makes thermostats, plugs and lights, but until now Hive has not supported HomeKit.
A lot of devices on the market support HomeKit now and finally, Hive does now too. However, you will need the newer Hive Hub and if your Hive Active Heating thermostat is controlling hot water, HomeKit will not work with it. Also, the Hive Wired Thermostat does not work with HomeKit either.
While the addition of HomeKit for Hive is good, there are better products which have more robust and complete intergration if you do want HomeKit, such as from Netatmo for heating and LIFX for lighting.