Havana means the act of offering oblations in fire. Homa has exactly the same meaning.
From Sanskrit Dictionary, few meanings of Havana are as follows:
हवन n. havana invocation
हवन m. havana god of fire
हवन n. havana summons
हवन n. havana challenging or challenge to
हवन n. havana act of offering an oblation with fire
Right now I am not able to find a reference where the word Havana is found. But, for Homa I can give a reference:
sandhyA snAnam japo homo devatAtithipujanam |
vaiswadevam cha shatkarmAni dine dine ||
Ablution and prayer, inaudible recitation (of sacred words), burnt-
offerings, the worship of gods, hospitality to guests unexpectedly
come, and offerings made in the name of the Visvadevam, — these are
the six duties to be performed every day
ParAshara Smriti 1.39
Yajna also has a similar meaning but it can imply more things besides fire sacrifices. It can mean sacrifice, offering etc.
Few meanings of Yajna are:
Sacrifice, act of worship or devotion, prayer, fire, worship,
So, while Havana/Homa has to be an offering made unto fire, Yajna is not necessarily the same. For example Japa Yajna - the act of doing Mantra Japa, here fire is not involved but still it's a Yajna.
Here is one such use of Yajna in scriptures where it is not referring to a sacrifice involving fire:
Japa Yajnath Paro Yajno NAparohastiha Kaschana ||
DharmArthaKAmaMokshanscha SAdhayeth ||
There is no Yajna which is superior to Japa Yajna, hence one should
attain Dharma, Artha, Kama & Moksha through Japa.
KulArnava Tantram,Chapter 15,Verse 3.
Some further references of how the word Yajna is used in Vedas are as follows:
Yat purushena havishA (1) deva yajnam atanvata (2) vasanto asyasid
Ajyam (3) grishma idhmah sharad havih (4) ||
With Purusha as the offering (1) the devas performed a sacrifice (2);
spring was the Ajya (3), summer the fuel (samit), and autumn (sharat)
the offering (4).
Now, here Yajna may not be a fire sacrifice, because fire or Agni was said to be born (in one of the subsequent Mantras) as a result of this sacrifice.
Yet another Mantra which uses the word Yajna in it. The meaning of this Mantra is quite difficult to comprehend in my opinion.
Yajnena yajnam ayajanta devAh (1) tAni dharmAni prathamAni Asan (2) te
ha nakam mahimAnah sachanta (3) yatra purve sAdhyAh santi devAh (4) ||
By sacrifice Gods sacrificed to sacrifice (1). These were the earliest
established principles (Dharma) (2). The Mighty Ones in this way
reached perfect bliss (3), Where dwell the Gods, ancients, who made
the Way straight (4).